BBC Washington correspondent Tom Carver looks at the background to impeachment

Members of the House of Representatives face what some have called the gravest task, apart from declaring war, that can ever face them - deciding on whether or not to impeach a president and send him to trial in the Senate.

Only once before in the history of the United States has the House impeached the occupant of the White House - 130 years ago when Andrew Johnson was sent to trial in the Senate. He escaped being convicted and removed from office by a single vote.

The only other time the House came close was in 1974 when Richard Nixon resigned after the House Judiciary Committee voted articles of impeachment against him but before the full House could vote and formally impeach him.

Richard Nixon came close to impeachment

But Bill Clinton has made no indication that he is going to take this route out.

Instead, through a mixture of apologies and discreet lobbying he is going to do what he has always done in his political career - keep fighting.

Impeachment would mark a new low for Bill Clinton's often troubled presidency.

One step from a trial

He had hoped to see this year as his last chance to make his mark as president and secure a place in the history books. He is now likely to achieve that but for all the wrong reasons.

Bill Clinton may survive but at what cost to his presidency?

If the House of Representatives approves any one of the four articles of impeachment before them by a simple majority, then Clinton is formally impeached, meaning he will have to stand trial before the Senate in January.

The Senate would probably take up the matter early in January. The President will face a trial - presided over by the Chief Justice of the United States - where the Senators will in effect make up the jury.

They will not be able to question witnesses directly, but can pass questions to Chief Justice Renquist, who will then put them to the witness.

Into the unknown

But before this painful process gets underway all sorts of procedural matters will have to be cleared away. Can, for example, the Senate proceed to a trial on a vote of the old House of Representatives?

Or, will the new House (elected at the mid-term elections last November) which has a smaller Republican majority, have to vote on impeachment all over again?

Constitutional experts frankly admit the road ahead is uncertain. But many fear if the Senate does indeed proceed with a trial, a precedent will have been set with important implications for the future relationship between president and Congress.

No-one alive today has ever been part of, or even seen, a Senate trial of the President. The scene would promise to be almost surreal with the 100 US Senators sitting silently as the jury, listening to the evidence against the president, in the company of the Chief Justice.

Showdown for the 'Comeback Kid'

When it finally comes to a vote a two-thirds majority would be needed to remove Clinton. That is almost certain not to happen and almost everyone believes Clinton will serve in office until the end of his term in January 2001.

But even so, a trial of the president would undoubtedly mesmerise and paralyse Washington and surely see the end of Clinton's ambitions.

Newt Gingrich: A surprise casualty

Every time Clinton seems to be down in his political career, whether it was in the 1992 New Hampshire primary when he was hit by draft dodging and sexual scandals; earlier this year when the Lewinsky scandal first broke; or back in 1980 when he lost the Governorship of Arkansas, he has always bounced back, earning himself the moniker the Comeback Kid.

When a few months ago the Lewinsky scandal reached fever pitch, no-one would have predicted that it would be Newt Gingrich, the Republican Speaker of the House, not Bill Clinton, who would have resigned and left office by the end of the year.