delivered the opinion of the Court.

Respondent Johnson, a former United States Congressman, was indicted and convicted on seven counts of violating the federal conflict of interest statute, 18 U. S. C. § 281 (1964 ed.),^[1] and on one count of conspiring to[*171] defraud the United States, 18 U. S. C. § 371 (1964 ed.).^[2] The Court of Appeals for the Fourth Circuit set aside the conviction on the conspiracy count, 337 F. 2d 180, holding that the Government’s allegation that Johnson had conspired to make a speech for compensation on the floor of the House of Representatives was barred by Art. I, § 6, of the Federal Constitution which provides that “for any Speech or Debate in either House, they [Senators and Representatives] shall not be questioned in any other Place.” The Court of Appeals ordered a new trial on the other counts, having found that the evidence adduced under the unconstitutional aspects of the conspiracy count had infected the entire prosecution.

The conspiracy of which Johnson and his three co-defendants were found guilty consisted, in broad outline, of an agreement among Johnson, Congressman Frank Boykin of Alabama, and J. Kenneth Edlin and William L. Robinson who were connected with a Maryland savings and loan institution, whereby the two Congressmen would exert influence on the Department of Justice to obtain the dismissal of pending indictments of the loan company and its officers on mail fraud charges. It was further claimed that as a part of this general scheme Johnson read a speech favorable to independent savings[*172] and loan associations in the House, and that the company distributed copies to allay apprehensions of potential depositors. The two Congressmen approached the Attorney General and the Assistant Attorney General in charge of the Criminal Division and urged them “to review” the indictment. For these services Johnson received substantial sums in the form of a “campaign contribution” and “legal fees.” The Government contended, and presumably the jury found, that these payments were never disclosed to the Department of Justice, and that the payments were not bona fide campaign contributions or legal fees, but were made simply to “buy” the Congressman.

The bulk of the evidence submitted as to Johnson dealt with his financial transactions with the other conspirators, and with his activities in the Department of Justice. As to these aspects of the substantive counts and the conspiracy count, no substantial question is before us. 18 U. S. C. § 371 has long been held to encompass not only conspiracies that might involve loss of government funds, but also “any conspiracy for the purpose of impairing, obstructing or defeating the lawful function of any department of Government.” Haas v. Henkel, 216 U. S. 462, 479. No argument is made, nor do we think that it could be successfully contended, that the Speech or Debate Clause reaches conduct, such as was involved in the attempt to influence the Department of Justice, that is in no wise related to the due functioning of the legislative process. It is the application. of this broad conspiracy statute to an improperly motivated speech that raises the constitutional problem with which we deal.^[3]

[*173] I.

The language of the Speech or Debate Clause clearly proscribes at least some of the evidence taken during trial. Extensive questioning went on concerning how much of the speech was written by Johnson himself, how much by his administrative assistant, and how much by outsiders representing the loan company.^[4] The government attorney asked Johnson specifically about certain[*174] sentences in the speech, the reasons for their inclusion and his personal knowledge of the factual material supporting those statements.^[5] In closing argument the[*175] theory of the prosecution was very clearly dependent upon the wording of the speech.^[6] In addition to questioning the manner of preparation and the precise in[*176] gredients of the speech, the Government inquired into the motives for giving it.^[7]

The constitutional infirmity infecting this prosecution is not merely a matter of the introduction of inadmissible evidence. The attention given to the speech’s substance and motivation was not an incidental part of the Government’s case, which might have been avoided by[*177] omitting certain lines of questioning or excluding certain evidence. The conspiracy theory depended upon a showing that the speech was made solely or primarily to serve private interests, and that Johnson in making it was not acting in good faith, that is, that he did not prepare or deliver the speech in the way an ordinary Congressman prepares or delivers an ordinary speech. Johnson’s defense quite naturally was that his remarks were no different from the usual congressional speech, and to rebut the prosecution’s case he introduced speeches of several other Congressmen speaking to the same general subject, argued that his talk was occasioned by an unfair attack upon savings and loan associations in a Washington, D. C., newspaper, and asserted that the subject matter of the speech dealt with a topic of concern to his State and to his constituents. We see no escape from the conclusion that such an intensive judicial inquiry, made in the course of a prosecution by the Executive Branch under a general conspiracy statute, violates the express language of the Constitution and the policies which underlie it.

II.

The Speech or Debate Clause of the Constitution was approved at the Constitutional Convention without discussion and without opposition. See Y Elliot’s Debates 406 (1836 ed.); II Records of the Federal Convention 246 (Farrand ed. 1911). The present version of the clause was formulated by the Convention’s Committee on Style, but the original vote of approval was of a slightly different formulation which repeated almost verbatim the language of Article V of the Articles of Confederation: “Freedom of speech and debate in Congress shall not be impeached or questioned in any court, or place out of Congress . . . .” The language of that Article, of which the present clause is only a slight modification, is in turn almost identical to the English Bill of Rights of 1689:[*178] “That the Freedom of Speech, and Debates or Proceedings in Parliament, ought not to be impeached or questioned in any Court or Place out of Parliament.” 1 W. & M., Sess. 2, c. 2.

This formulation of 1689 was the culmination of a long struggle for parliamentary supremacy. Behind these simple phrases lies a history of conflict between the Commons and the Tudor and Stuart monarchs during which successive monarchs utilized the criminal and civil law to suppress and intimidate critical legislators.^[8] Since the Glorious Revolution in Britain, and throughout United States history, the privilege has been recognized as an important protection of the independence and integrity of the legislature. See, e. g., Story, Commentaries on the Constitution § 866; II The Works of James Wilson 37-38 (Andrews ed. 1896). In the American governmental structure the clause serves the additional function of reinforcing the separation of powers so deliberately established by the Founders. As Madison noted in Federalist No. 48:

“It is agreed on all sides, that the powers properly belonging to one of the departments, ought not to be directly and compleatly administered by either of the other departments. It is equally evident, that neither of them ought to possess directly or indirectly, an overruling influence over the others in the administration of their respective powers. It will not be denied, that power is of an encroaching nature, and that it ought to be effectually restrained from passing the limits assigned to it. After discriminating therefore in theory, the several classes of power, as they may in their nature be legislative,[*179] executive, or judiciary; the next and most difficult task, is to provide some practical security for each against the invasion of the others. What this security ought to be, is the great problem to be solved.” (Cooke ed.)

The legislative privilege, protecting against possible prosecution by an unfriendly executive and conviction by a hostile judiciary, is one manifestation of the “practical security” for ensuring the independence of the legislature.

In part because the tradition of legislative privilege is so well established in our polity, there is very little judicial illumination of this clause. Clearly no precedent controls the decision in the case before us. This Court first dealt with the clause in Kilbourn v. Thompson, 103 U. S. 168, a suit for false imprisonment alleging that the Speaker and several members of the House of Representatives ordered the petitioner to be arrested for contempt of Congress. The Court held first that Congress did not have power to order the arrest, and second that were it not for the privilege, the defendants would be liable. The difficult question was whether the participation of the defendants in passing the resolution ordering the arrest was “speech or debate.” The Court held that the privilege should be read broadly, to include not only “words spoken in debate,” but anything “generally done in a session of the House by one of its members in relation to the business before it.” 103 U. S., at 204.

In Tenney v. Brandhove, 341 U. S. 367, at issue was whether legislative privilege protected a member of the California Legislature against a suit brought under the Civil Rights statute, 8 U. S. C. §§ 43, 47 (3) (1946 ed.), alleging that the legislator had used his official forum “to intimidate and silence plaintiff and deter and prevent him from effectively exercising his constitutional[*180] rights of free speech and to petition the Legislature for redress of grievances . . . 341 U. S., at 371. The Court held a dismissal of the suit proper; it viewed the state legislative privilege as being on a parity with the similar federal privilege, and concluded that

“The claim of an unworthy purpose does not destroy the privilege-.. .. The holding of this Court in Fletcher v. Peck, 6 Cranch 87, 130, that it was not consonant with our scheme of government for a court to inquire into the motives of legislators, has remained unquestioned.” 341 U. S., at 377.

III.

Kilbourn and Tenney indicate that the legislative privilege will be read broadly to effectuate its purposes; neither case deals, however, with a criminal prosecution based upon an allegation that a member of Congress abused his position by conspiring to give a particular speech in return for remuneration from private interests. However reprehensible such conduct may be, we believe the Speech or Debate Clause extends at least so far as to prevent it from being made the basis of a criminal charge against a member of Congress of conspiracy to defraud the United States by impeding the due discharge of government functions. The essence of such a charge in this context is that the Congressman’s conduct was improperly motivated, and as will appear that is precisely what the Speech or Debate Clause generally forecloses from executive and judicial inquiry.

Even though no English or American case casts bright light on the one before us^[9] it is apparent from the history[*181] of the clause that the privilege was not born primarily of a desire to avoid private suits such as those in Kil-bourn and Tenney, but rather to prevent intimidation by the executive and accountability before a possibly hostile judiciary. In the notorious proceedings of King Charles I against Eliot, Hollis, and Valentine, 3 How. St. Tr. 294 (1629), the Crown was able to imprison members of Commons on charges of seditious libel and conspiracy to detain the Speaker in the chair to prevent adjournment.^[10] Even after the Restoration, as Holds-worth noted, “[t]he law of seditious libel was interpreted with the utmost harshness against those whose political or religious tenets were distasteful to the government.” VI Holdsworth, A History of English Law 214 (1927). It was not only fear of the executive that caused concern in Parliament but of the judiciary as well, for the judges were often lackeys of the Stuart monarchs,^[11] levying punishment more “to the wishes of the crown than to the[*182] gravity of the offence.” Id,, at 214-215. There is little doubt that the instigation of criminal charges against critical or disfavored legislators by the executive in a judicial forum was the chief fear prompting the long struggle for parliamentary privilege in England and, in the context of the American system of separation of powers, is the predominate thrust of the Speech or Debate Clause. In scrutinizing this criminal prosecution, then, we look particularly to the prophylactic purposes of the clause.^[12]

The Government argues that the clause was meant to prevent only prosecutions based upon the “content” of speech, such as libel actions, but not those founded on “the antecedent unlawful conduct of accepting or agreeing to accept a bribe.” Brief of the United States, at 11. Although historically seditious libel was the most frequent instrument for intimidating legislators, this has never been the sole form of legal proceedings so employed,^[13] and the language of the Constitution is framed[*183] in the broadest terms. The broader thrust of the privilege is indicated by a nineteenth century British case, Ex parte Wason, L. R. 4 Q. B. 573 (1869), which dealt specifically with an alleged criminal conspiracy. There a private citizen moved that a magistrate be required to prosecute several members of the House of Lords for conspiring wrongfully to prevent his petition from being heard on the floor. The court denied the motion, stating that statements made in the House “could not be made the foundation of civil or criminal proceedings . . . . And a conspiracy to make such statements would not make the person guilty of it amenable to the criminal law.” Id., at 576. (Cockburn, C. J.) Mr. Justice Lush added, “I am clearly of opinion that we ought not to allow it to be doubted for a moment that the motives or intentions of members of either House cannot be inquired into by criminal proceedings with respect to anything they may do or say in the House.” Id., at 577.

[*184] In the same veip. the Government contends that the Speech or Debate Clause was not violated because the gravamen of the count was the alleged conspiracy, not the speech, and because the defendant, not the prosecution, introduced the speech itself.^[14] Whatever room the Constitution may allow for such factors in the context of a different kind of prosecution, we conclude that they cannot serve to save the Government’s case under this conspiracy count. It was undisputed that Johnson delivered the speech; it was likewise undisputed that Johnson received the funds; controversy centered upon questions of who first decided that a speech was desirable, who prepared it, and what Johnson’s motives were for making it. The indictment itself focused with particularity upon motives underlying the making of the speech and upon its contents:

“(15) It was a part of said conspiracy that the said THOMAS F. JOHNSON should . . . render services, for compensation, ... to wit, the making of a speech, defending the operations of Maryland’s ‘independent’ savings and loan associations, the financial stability and solvency thereof, and the reliability and integrity of the ‘commercial insurance’ on investments made by said ‘independent’ savings and loan associations, on the floor of the House of Representatives.” App. 5-6.

We hold that a prosecution under a general criminal statute dependent on such inquiries necessarily contra[*185] venes the Speech or Debate Clause. We emphasize that our holding is limited to prosecutions involving circumstances such as those presented in the case before us. Our decision does not touch a prosecution which, though as here founded on a criminal statute of general application, does not draw in question the legislative acts of the defendant member of Congress or his motives for performing them. And, without intimating any view thereon, we expressly leave open for consideration when the case arises a prosecution which, though possibly entailing inquiry into legislative acts or motivations, is founded upon a narrowly drawn statute passed by Congress in the exercise of its legislative power to regulate the conduct of its members.^[15]

The Court of Appeals’ opinion can be read as dismissing the conspiracy count in its entirety. The making of the speech, however, was only a part of the conspiracy charge. With all references to this aspect of the conspiracy eliminated, we think the Government should not be precluded from a new trial on this count, thus wholly purged of elements offensive to the Speech or Debate Clause.

IV.

The Court of Appeals held that Johnson was entitled to a new trial on the conflict of interest counts because the admission of evidence concerning the speech aspect of the conspiracy count was prejudicial on these other counts as well. The Government reserved the right to contest the order of a new trial, but, except for a footnote in its reply brief, it did not so argue in this Court; on the contrary it stated in oral argument that it stood solely on its position with reference to the conspiracy[*186] count.^[16] In these circumstances we find no occasion to review the Court of Appeals’ assessment of the record in this respect.

For the foregoing reasons we affirm the judgment of the Court of Appeals and remand the case to the District Court for further proceedings consistent with this opinion. jf- is so ordered.