The Secret Service agents closest to President Clinton have been summoned to testify as part of the investigation about his relationship with the former White House intern, Ms Monica Lewinsky.
The independent counsel, Mr Kenneth Starr, has also demanded detailed Secret Service records about the president's whereabouts on nights when Ms Lewinsky was known to be at the White House.
The action by Mr Starr, to subpoena up to a dozen agents, has angered the White House, which sees it as a "backdoor attempt" to learn about the president's confidential conversations.
These agents are the armed plainclothes men who are constantly at the president's side. Among those summoned to appear before the grand jury today is Mr Larry Cockell, the special agent in charge of the White House detail, who is scheduled to fly with Mr Clinton to Arkansas tomorrow.
Up to now Mr Starr has been seeking the testimony only of the uniformed agents who guard access to the Oval Office and the grounds of the White House.
The Department of Justice immediately tried to quash the subpoenas in a secret hearing in a federal court.
Mr Starr's latest move follows a decision by the federal appeals court to reject a claim by the Department of Justice that the Secret Service is covered by a "protective privilege" from having to testify. According to this argument, presidents would keep Secret Service agents at a distance if they thought their private conversations could later be reported by the agents.
The White House press secretary, Mr Mike McCurry, said yesterday that the Secret Service was now being "subpoenaed by an over-zealous prosecutor to betray their trust". Mr McCurry said that the president assumed his conversations were confidential, and that the agents who were present should not be made testify.
Mr Clinton's personal lawyer, Mr David Kendall, said that any backdoor attempt by Mr Starr to breach lawyer-client privilege would be "firmly and aggressively resisted".
This is a reference to the fact that the Secret Service head agent, Mr Cockell, accompanied Mr Clinton and his lawyer, Mr Bob Bennett, on the day the president testified to the lawyers of Ms Paula Jones about his relations with Ms Lewinsky.
It is likely that the arguments over "protective privilege" for the Secret Service will end up in the Supreme Court. But this would mean that no final decision could be made until the autumn at the earliest.
Republicans on Capitol Hill are accusing President Clinton and his aides of deliberately delaying Mr Starr's investigation by a series of appeals to the courts on claims of privilege for witnesses called to testify before the grand jury.
The White House alleges that Mr Starr is politically motivated and has dragged out his original Whitewater inquiry over five years at enormous expense to the taxpayer without coming up with any evidence of wrong-doing by Mr Clinton or his wife, Hillary.
Meanwhile, Ms Linda Tripp has been continuing her secret testimony to the grand jury this week about her taping of conversations with Ms Lewinsky which has led to the present investigation. Ms Tripp is herself facing possible indictment by another grand jury for illegally taping telephone conversations.