This week sees the battle between the Clinton presidency and the relentless independent counsel, Mr Kenneth Starr, move into a critical phase as he at last gets to question the men he believes have information vital for his investigation.

It has taken Mr Starr six months of legal battles to get the Secret Service agents who guard the President to testify under oath to a grand jury investigating whether Mr Clinton committed perjury over his relationship with former White House intern, Ms Monica Lewinsky, or tried to get others to commit perjury. Starr already has amassed thousands of pages of evidence but needs the testimony of the agents to fill out the overall picture.

In his brief to the Supreme Court arguing that the agents cannot refuse to testify, Mr Starr said his office "is in possession of information that Secret Service personnel may have observed evidence of possible crimes while stationed in and around the White House complex."

Ms Lewinsky is said to have been hastily transferred from the White House when a Secret Service agent warned a senior aide about some aspects of her behaviour.

The agents will be questioned whether they ever saw the President in compromising positions with Ms Lewinsky and whether she was ever alone with him in the White House. The President has said he cannot recall being alone with her.

Because the Secret Service put up such a fierce resistance to being questioned under oath, there has naturally been speculation that there must be something to hide. This may not be the case.

The Secret Service lawyers argued it was the security of future presidents as well as this one which was at stake if the agents could be questioned about the movements and conversations of the person whose life they swear to protect with their own. They are ready "to take a bullet" for the president.

Now they fear presidents will keep them at a distance and so compromise their safety from assassination attempts, but none of the courts has accepted this "protective function privilege".

How dangerous is this development for Mr Clinton? He has given a sworn statement to lawyers of Ms Paula Jones that he did not have sexual relations with Ms Lewinsky and he has said this publicly to the American public twice on television. He has also denied he tried to get anyone to commit perjury as part of a cover-up.

Mr Starr's official mandate is to make a report to Congress if he obtains evidence that the President has committed acts for which he could be impeached. Having an affair with Ms Lewinsky or anyone else would not fall under this heading. While it would be adultery, it would not be a felony.

However, if Mr Starr ends up with evidence that Mr Clinton committed perjury or "suborned" others to do so, then Congress would probably feel obliged to start the impeachment process. The danger for Mr Clinton is that having sworn he did not have sexual relations with Ms Lewinsky, he would be seen as a perjurer if evidence is produced that he did have such an affair.

Ms Lewinsky has also sworn there was no improper relationship although she has been taped by a former colleague, Linda Tripp, allegedly admitting such an affair lasting over 17 months.

Here is another danger for Mr Clinton. She could be indicted by Mr Starr on charges of perjury if he has firm evidence that she lied under oath. To avoid such an indictment and a possible prison sentence, Ms Lewinsky may admit the perjury in exchange for immunity from prosecution.

Mr Starr would only give her this immunity in exchange for evidence about the subject he is really pursuing - did Mr Clinton, his confidant Vernon Jordan, or his White House counsellor Bruce Lindsey, try to get Ms Lewinsky to lie under oath about an affair?

It is widely believed Ms Lewinsky, through her former lawyer, William Ginsburg, was prepared to testify that she did have sex with Mr Clinton but would deny he asked her to lie under oath about it. This was not enough for Mr Starr who is sitting on pages of secret testimony he has gathered from others over the past six months.

As Mr Starr wraps up his questioning this week of Linda Tripp and interrogates the Secret Service agents, he will be ready to confront Ms Lewinsky with an intimidating body of evidence which he hopes will persuade her to co-operate fully with him or else face a possible prison term for contempt of court.

Her new highly experienced Washington lawyers will do all they can to keep her out of jail even if this means trouble for Mr Clinton.

The strain of all this shows at times on the President, Hillary Clinton and the press secretary, Mr Mike McCurry, as a White House press corps scenting blood yaps at their heels. The Clintons have learned to "compartmentalise", to use their own term, to allow them to carry on every day with their official duties.

Foreign trips have also helped and the September visit to Russia and almost certainly to Ireland will be welcomed as relief from Mr Starr.

Does the US outside the Washington Beltway care much about all this as the economy roars ahead and the Dow Jones index climbs steadily? Polls show most people have heard enough about Monica Lewinsky and what she might or might not have done in the Oval Office. That's the Clintons' business.

However, if Mr Starr comes up at some stage with clear evidence that the President and his top aides were engaged in a cover-up which involved perjury and finding Ms Lewinsky a lucrative job to buy her silence, that's another story.

The Republican-controlled Congress would start the impeachment process and even Democrats would falter in support for the President if the evidence were damning.

If Mr Starr produces a damaging report before the mid-term congressional elections next November, Monica Lewinsky's activities in the White House will be part of the election campaign. This could destroy any chance the Democrats now have of winning the 11 seats to give them back control of the House of Representatives.

If Mr Starr holds back until after the election, Mr Clinton will be into the "lame duck" phase of his two-term presidency and seen as increasingly irrelevant as the Vice-President, Al Gore, tries to tie up the Democratic nomination and the Republicans search for the candidate who could beat him.

Mr Clinton, however, still hungers for a "legacy" to mark the first two-term Democratic presidency since Franklin D. Roosevelt. He does not want Monica Lewinsky going into the history books with him.