Washington's establishment, including the mainstream media, has been feverishly anticipating Mr Clinton's testimony for the best part of a month. From the time he enters the White House Map Room to begin his evidence on closed circuit TV to the court, to when he addresses the nation (as he is fully expected to do), the political world will be on hold.
The White House has been in crisis mode for three weeks, since the former White House trainee, Monica Lewinsky, agreed terms with the independent prosecutor, Kenneth Starr, for giving her version of her relationship with the President.
Few, if any, White House staff have been privy to the President's thinking. Partly this is because it would have exposed them to the risk of being called to account before the grand jury. It is partly also because Mr Clinton is not in the habit of confiding in others on such matters, except perhaps his wife.
As the White House spokesman, Mike McCurry, said soon after the first allegations broke, the relationship must be "complicated", why else would he not have explained it? Some 70 per cent of Americans now tell pollsters that they believe Mr Clinton had an affair with Ms Lewinsky, and lied about it. That number has risen steadily since January.
Until now, an only a slightly smaller majority (63 per cent) have said they approve of how Mr Clinton is doing his job. Depending on how Mr Clinton testifies, that public support could change. The media consensus is that if Mr Clinton goes back on his earlier denials, the reaction may be unpredictable. "It could alter the President's whole relationship with the American people," said one commentator.
Mr Clinton's difficulties in the Lewinsky matter date back to sworn testimony he gave on 17 January to lawyers in the sexual harassment case brought by former Arkansas state employee Paula Jones.
While hedging and fudging most of his replies, Mr Clinton expressly denied a sexual relationship with Ms Lewinsky. Four days later, the American media published allegations, then transcribed excerpts from tape-recordings, claiming Ms Lewinsky had had an affair with the President. Twice in the next week, Mr Clinton used TV to tell America that there had been no sexual relationship.
Today, Mr Clinton must face the same grand jury - 23 randomly selected individuals, of whom most are reported to be women and black - that has already heard evidence from his staff and Monica Lewinsky. The most obviously safe option, personally and for his presidency, would be for him to stick to his earlier denials of a sexual relationship with Ms Lewinsky.
That only works if the denial was true, and/or if there is no evidence to contradict it. All the twisting and turning from White House "sources" in recent days suggests this option is closed. A denial before a federal grand jury that was subsequently proved to be false would put Mr Clinton on a direct route to impeachment.
This leaves him with having to decide how much to admit, and how. A dramatic outpouring of guilt and repentance appears to have been rejected. However kindly disposed the American public might be to a penitent President, a confession would leave Mr Clinton open to a charge of perjury relating to his testimony in the Paula Jones case. Opinions differ as to whether that would be serious enough to warrant impeachment proceedings.
The favoured option seems to be a middle way steered between admitting some form of relationship and denying perjury. This might let him off some legal hooks, but the political - and private - difficulties would remain.
How would American public opinion receive such a partial admission in the light of his televised: "I did not have sexual relations with that woman, Ms Lewinsky". "Sex is sex," said Ann Lewis, the White House communications director, after the first allegations broke, and Americans seem to agree. They might find legalistic definitions of what constitutes sexual relations more off-putting than a direct confession.
Hillary Clinton could also be embarrassed because she said in January that "it would be a very serious offence" if "a President" were proved to have had an adulterous affair in the White House, and lied about it.
This is why, despite the American public's professed distaste for the Lewinsky saga, the affair known as "Zippergate" cannot be laughed off. Those who draw judicial comparisons with Watergate and political parallels with Vietnam are not exaggerating.
What Mr Clinton says today, and how America reacts tomorrow, will determine his own, and the country's future.
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