The most important - and enigmatic - figure in President Clinton's impeachment trial will be the man charged with overseeing the historic proceedings: Mr William Rehnquist, the Chief Justice of the Supreme Court.
The nominal chairman of the Senate is the Vice-President, Mr Al Gore, but the US founding fathers deemed it inappropriate that the president's potential successor should chair proceedings that could lead to his own accession.
At first glance this would appear to be bad news for Mr Clinton. Mr Rehnquist is a Republican who was appointed to the court by Mr Richard Nixon, the last president to face impeachment. He became Chief Justice in 1986 and during his long tenure has consolidated a reputation as a tough but fair-minded jurist with a strong conservative leaning.
But paradoxically, Mr Rehnquist's attitude to the constitution and its insistence on the separation of powers means he may prove surprisingly sympathetic to Mr Clinton's cause.
Although there are few guides as to what actions deserve impeachment and how such a trial should be structured, Mr Rehnquist has long taken a personal interest in the topic. In 1992 he wrote a book examining the only precedent: the trial of the Democrat president, Andrew Johnson, by a partisan, Republican-dominated Congress in 1868.
Johnson was accused of violating an ambiguous law concerning his authority to make cabinet appointments, although his real crime in the eyes of Republicans was opposing the tough line against the defeated southern states promoted by Congress.
Examining those events, Mr Rehnquist concluded that the Senate had been correct to absolve Johnson of wrongdoing (he survived by one vote).
Mr Rehnquist's book warned that if Congress approved impeachment of a sitting president along purely partisan lines it could violate the independence of the executive branch of government, operating like a "sword of Damocles" hanging above the presidency.
He is also on record as stating that impeachment should be confined to charges of "flagrant abuse of office - perjury, bribery and the like". However, it is not clear whether he regards the perjury charges in the articles of impeachment against Mr Clinton, which occurred in a civil suit, as having legal merit. Whatever the shape of the trial, one issue appears certain. The proceedings will dominate public life for their duration.
In an interview shortly after the publication of his book on the Johnson impeachment, Mr Rehnquist said: "You can tell from the publicity that the newspapers of the day gave to the Johnson impeachment that nothing much happened in the presidency between the time that Johnson was impeached in February and the time he was acquitted in May.
"It just really significantly impairs, if it doesn't cripple, a president to be the subject of an impeachment trial. It also takes up a tremendous amount of time on the part of Congress so that Congress is not able to do very much. It's just a very time-consuming procedure."