Article I, Section 6 contains several clauses of great significance. It begins by specifying that Senators and Representatives shall be paid out of the Treasury of the United States and not by the states they represent as had been the practice under the Articles of Confederation. This emphasizes the fact that Senators and Representatives are officers of the federal government and not of the state wherein they were elected.
Article I, Section 6 also says that Senators and Representatives shall not be questioned in court or by the President for any speech or debate they give or participate in on the floor of the Senate or the House. This assures ample freedom of debate in Congress.
Article I, Section 6 provides in addition that Senators and Representatives cannot be appointed to offices the salaries of which had been increased during the time of their service in Congress. This Ineligibility Clause is a simple ethics rule preventing Members of Congress from increasing the salary of an executive office and then soliciting the President to appoint them to that office so they can benefit from the salary increase they had voted for.
An interesting question raised by this Clause is whether a Senator or Representative who was serving in Congress when the salary of an executive office was raised may be appointed to that office if the salary of the office is subsequently reduced to its original level. This scenario has occurred several times in relatively recent history. The first time it came up, the question was whether President Richard Nixon could appoint Senator William Saxbe to be his Attorney General, even though Congress had raised that office’s salary while Saxbe was a Senator (after the salary was rolled back to its original level, Saxbe did in fact take office). The so-called “Saxbe fix” came up again in 1987, when President Ronald Reagan was considering whether he could appoint Senator Orrin Hatch to the Supreme Court even though the Justices’ salaries had been raised while Hatch was a Senator. In that instance, the Office of Legal Counsel at the Department of Justice issued an opinion declaring that the Saxbe fix would not render the appointment constitutional. More recently, however, the same office at the Justice Department reached the opposite conclusion, thus clearing the way for President Barack Obama to appoint Hillary Clinton to be the Secretary of State despite the fact that the Secretary’s salary had been increased during Clinton’s tenure in the Senate. See Justice Department Memorandums for the Attorneys General.
Finally, and most importantly, Article I, Section 6 provides that “no person holding any office under the United States, shall be a member of either House during his continuance in office.” This provision is of profound structural importance since it prevents the appointment of powerful Senators or Congressmen to the President’s Cabinet thus reinforcing the separation of legislative and executive power. Absent this Incompatibility Clause, it is quite likely that powerful Senators and Representatives would demand that they be appointed to the Cabinet, and the President would likely have to acquiesce. This would create a sort of parliamentary government in which the President was independent but all the great officers of state were Congressmen. Thanks to the Incompatibility Clause, the U.S. has a separation of powers and a separation of personnel. The same people cannot simultaneously exercise legislative and executive (or judicial) power. Members of Congress are barred from holding executive branch or judicial offices. For further discussion of this Clause, see Steven G. Calabresi & Joan L. Larsen, One Person, One Office: Separation of Powers or Separation of Personnel?, 79 Cornell L. Rev. 1045-1157 (1994); Jay Wexler, The Odd Clauses: Understanding the Constitution Through Ten of its Most Curious Provisions, ch.1 (2012).
Occasionally, the question arises—what counts as an “office under the United States”? It has been suggested by one scholar, for instance, that the office of President is not such an office because the President presides over the executive branch rather than being an officer within it. If this scholar were right (and neither of us think that he is), then it would follow that the Speaker of the House of Representatives and President Pro Tempore of the Senate could serve as President without violating the Incompatibility Clause. And what about members of the National Guard or military reserve officers? Can a member of Congress simultaneously serve in the reserves? In the 1970s, a federal court faced with this question held that such an arrangement violated the Incompatibility Clause, but the Supreme Court subsequently dismissed the case on grounds that the plaintiffs lacked standing to raise the question. The issue, therefore, remains unresolved. See Schlesinger v. Reservists to Stop the War (1974).
The Framers added the Incompatibility Clause to the Constitution as a constitutional ethics rule, and no one at the Founding foresaw the profound structural effects it would have on American government. Eighteenth century British Monarchs had bribed their way to control over the House of Commons by offering Members of the House lucrative executive offices to secure the Members’ votes on issues important to the King. The Framers of the U.S. Constitution were repulsed by this behavior, and so they added the Incompatibility Clause to the Constitution as an ethics rule to prevent such corruption in the United States. An unintended consequence of the Framers’ incompatibility ethics rule was that it empowered the President greatly by making it unconstitutional for powerful Members of Congress to demand Cabinet appointments.
There is no Incompatibility Clause for executive and judicial officers, and there is thus a long history of judges holding executive branch as well as judicial offices. Chief Justice John Jay thus negotiated a peace treaty with Great Britain as an ambassador; Chief Justice John Marshall served simultaneously as Secretary of State and as Chief Justice in February 1801; Justice Robert Jackson was the Chief Allied Prosecutor of the Nazis at the Nuremberg Trials; and Chief Justice Earl Warren served as the head of an executive branch Commission—the Warren Commission—which investigated who had been involved in the assassination of President John F. Kennedy. The American Bar Association frowns on such joint judicial and executive branch office-holding for obvious ethical reasons, and no recent Justice has held any executive branch office.
There is also no federal constitutional Incompatibility Clause forbidding the joint holding of federal and of state offices. Many state constitutions, however, do forbid this. Most modern constitutions have strict Incompatibility Clauses based on a sensible One Person, One Office principle.
The Incompatibility Clause of Article I, Section 6 may be under inclusive, but it is perhaps the foremost reason why the U.S. never developed a parliamentary system of government. Because Members of Congress are forbidden from holding executive offices, they have instead set up a very powerful congressional committee system, which shadows each presidential Cabinet department. The Senate and House Foreign Affairs Committees thus shadow the State Department just as the Senate and House Judiciary Committees shadow the Justice Department. The congressional committee system is the stunted growth of a parliamentary government in the United States, which cannot take over the Executive Branch because the Incompatibility Clause prevents Congressional Committee Chairmen from demanding Cabinet appointments. Accordingly, the Incompatibility Clause guarantees an independent executive branch headed up by the President, and it is one of the most important, even if unnoticed, clauses in the Constitution.