We like the separation of powers because it serves as a bar against undue executive or legislative or judicial presumption. The point of the American system, as the great justice of the Supreme Court, Louis D Brandeis, put it, is "not to promote efficiency but to preclude the exercise of arbitrary power". The Constitution thus institutionalizes conflict in the very heart of the American polity.
Conflict between the executive and the legislative branches often takes place, even when the same political party controls both branches. And, latterly, the constitutional separation of powers has been reinforced by a political separation of powers. Since the Second World War, American voters have become increasingly fond of electing a president of one party and a Congress controlled wholly or partly by the opposition.
This taste for divided government is no doubt a reaction against so-called Imperial Presidency - the enhanced power flowing to the president to protect the republic from dangers abroad. In the half century from 1939 to 1989, the United States was in a condition of protracted international crises - crises that encouraged Congress to surrender power to the executive, especially the power to go to war.
Once international crisis receded, Congress began to reclaim its powers. This tends to happen after every war. Woodrow Wilson and the First World War were followed by Warren G Harding and his "return to normalcy". After the Second World War, the Republicans got their posthumous revenge against Franklin D Roosevelt by securing an amendment to the Constitution denying all future presidents more than two terms in the White House.
Most spectacular was the impeachment of Andrew Johnson after the American Civil War. "Impeachment", by the way, is equivalent to indictment. All it means is that the House of Representatives votes to send a case to the Senate, which then must decide by two-thirds vote whether or not the official is guilty, in the words of the Constitution, of "treason, bribery or other high crimes and misdemeanors", and is to be removed from office. When President Johnson fired his Secretary of War in violation of a so- called "tenure of office act" passed by Congress, the House voted to impeach Johnson. The senate then acquitted him by a single vote.
The president may have been rescued in 1868, but the presidency was damaged. One senator said: "Whether Andrew Johnson should be removed from office, justly or unjustly, was comparatively of little consequence - but whether our government should be Mexicanized, and an example set which would surely, in the end, utterly overthrow our institutions, was a matter of vast consequence." James G Blaine, a formidable Republican leader of the period, had voted for impeachment in the House; but, reflecting 20 years later, Blaine wrote that the success of the impeachment drive "would have resulted in greater injury to free institutions than Andrew Johnson in his utmost endeavour was able to inflict".
The aftermath bound and confined the presidency for the rest of the century. Woodrow Wilson, then a young political scientist, decided that Congress had become "the central and predominant power of the system", and entitled his brilliant and influential book, Congressional Government.
Will something like this happen today? Is the United States in for another experiment in congressional government? No one at this point can foretell the next chapters in Mr Clinton's Hogarthian saga, but they are not likely to be happy ones. Kenneth Starr, now unveiled as the nation's number one pornographer, is far more widely despised than Mr Clinton, but Mr Starr could not have polluted the Internet without Mr Clinton's collaboration.
The House Judiciary Committee will hold hearings on the Starr report, for the Republican leadership hopes to extract the maximum political benefit from the President's disgrace. This effort could even acquire a life of its own and lead to the submission of articles of impeachment to the full House.
Perhaps Mr Starr has further cards up his sleeve; but, at present, indictment of the President derives entirely from Mr Clinton's lies about his sex life. Of course, most people at one time or another lie about their sex lives. Only a cad tells the truth about his love affairs. It seems doubtful that the Senate would terminate a presidency because of such lies, even told under oath. Impeachment, most people feel, should be reserved for gross abuses of official authority. So, unless there are damaging new revelations, successful impeachment seems unlikely.
Mr Clinton is not likely to resign. He would regard resignation as vindication of the despised Mr Starr. Moreover, polls continue to show that most Americans want him to continue in public office. Most Americans also feel the need to register disapproval of his private behaviour. The obvious solution would be a resolution of censure.
Only once before has a president been censured. In 1834, the Senate censured Andrew Jackson for his transfer of public funds from the Bank of the United States to state banks. When the Democrats regained control of the Senate, the resolution of censure was expunged from the record.
One can expect a severely diminished presidency for the immediate future. Mr Starr's aggrandizement of the role of special prosecutor has already imposed extraordinary restraints on the presidency. He has obtained, mostly from Republican judges, rulings that turn White House lawyers and aides, as well as Secret Service personnel, into informers for the prosecutor. It is now hard to see with whom presidents can freely discuss anything - except for their wives, who cannot be compelled to testify against their husbands.
But future Congresses can remedy these matters. Even Republicans will acquire a new perspective on the presidency when they hope to recapture the White House. The special prosecutor act itself, due to expire next year, will, if renewed, very likely include restrictions on time, budget and jurisdiction designed to prevent protracted, freewheeling, dragnet investigations on the Starr model.
For the American presidency is indestructible. A system based on the tripartite separation of powers has an inherent tendency toward stalemate. One of the branches must take the initiative if the system is to move at all. The executive branch alone is structurally capable of taking that initiative. A strong presidency remains the key to the American system.
Mr Clinton's disgrace does not nullify the constitutional and institutional powers of the presidential office, or the president's capacity to decide policies and set goals. Mr Clinton, moreover, is an escape artist of the first water. If he wishes to recover a place in history, let him fight hard for the lofty ideals that he brought to the White House: for education, health care and social security, against the role of money in politics and against the increasing inequality of wealth and income. He may lose such fights, but he will educate the electorate in the issues and lay the foundation for more reforms in the future.
Strong presidents have always lived risky lives. As Charles Dickens told the American people after visiting the United States a century and a half ago: "You no sooner set up an idol firmly than you are sure to pull it down and dash it into fragments... Any man who attains a high place among you, from the President downwards, may date his downfall from that moment." That seems to be the American way.
The author, a celebrated historian of the American presidency, was an adviser to John F Kennedy