As Republican prosecutors begin to wind up their case against President Clinton, the pressure is increasing on Democrats to agree to witnesses being called later in the trial.
Some Republicans are suggesting that Mr Clinton be called as a witness, but the White House is dismissing the idea.
The prosecution case will conclude today with constitutional arguments by the Republican managers from the House of Representatives to show that the charges of perjury and obstruction of justice against the President warrant his removal from office.
The President's legal team will begin presenting his defence on Tuesday, following adjournment for the weekend and the Martin Luther King public holiday. On Tuesday night President Clinton is due to deliver his State of the Union address to a joint session of the House and Senate.
First reactions to the opening day of the trial were mixed, with some Democratic senators saying they heard nothing new. But some Republicans said they could now see why witnesses should be called to help to establish whether Mr Clinton lied under oath about his relationship with Ms Lewinsky and obstructed justice in the Paula Jones civil suit for sexual harassment.
Democrats have expressed annoyance at what they see as a breach of an agreement with Republicans last week that the whole question of witnesses be put off until a later stage, after the prosecution and defence cases have been heard and the senators who act as jurors have asked questions.
The Democratic criticism came when it was revealed that some Republican senators have been meeting secretly with the prosecution team to discuss what witnesses should be called. Senator Tom Daschle, the Democratic leader, said that such a meeting "certainly violates the spirit of the agreement we all agreed to last week".
A new poll by NBC News/Wall Street Journal shows that opposition by the public to the removal of the President is decreasing as the trial starts. The latest figure of 57 per cent opposed to removal and 36 per cent in favour is said to be a significant shift.
The independent pollster Mr John Zogby said that the number for removal was up by 10 per cent and opposition was down by 11 per cent. "I think what could happen is that the more it plays out in the Senate, the more you can see support for removal, especially since the Senate's tone in this proceeding is so much more moderate and benign than it was in the House," Mr Zogby said.