A visibly shaken Marcia Lewis completed a second day of grand jury testimony about her daughter's ties to President Clinton yesterday and was forced to return again today, as Monica S. Lewinsky's own appearance before the panel was postponed.

Lewis testified behind closed doors for more than 4 1/2 hours but by midafternoon was so overcome with emotion that the courthouse nurse was briefly summoned and she was unable to continue answering questions. Her bottom lip quivering, Lewis walked out of the courthouse under her own power, escorted by court security officers.

"This is a very emotionally draining and difficult time for my client," Lewis's lawyer, Billy Martin, said in a later telephone interview. "No mother should be forced by federal prosecutors to testify against her own child."

Neither Lewis nor Martin would discuss what specifically upset her so much yesterday, but the appearance before the grand jury has developed into a far more protracted event than anticipated. Lewis, who has been forced to testify against her will under a grant of limited immunity, will undergo an unexpected third day straight of interrogation, effectively putting off Lewinsky's session with prosecutors and the grand jury, which originally was scheduled for today.

Lewinsky, 24, shared an apartment at the Watergate with her mother and confided in her regularly, according to people close to the family. Prosecutors working for independent counsel Kenneth W. Starr want to know whether Lewis, 49, an author who shortened her name, was aware of any sexual relationship between her daughter and Clinton and whether Lewinsky was urged by the president to deny it under oath, as the former White House intern and clerk reportedly told a friend in secretly recorded conversations.

As the Lewis testimony dragged on, the White House moved to defend the president on several fronts. In a session with reporters, first lady Hillary Rodham Clinton insisted that the crisis that has endangered her husband's presidency "will slowly dissipate." Clinton advisers also criticized a story in yesterday's Washington Post in which a retired Secret Service officer said that the president had spent at least 40 minutes alone with Lewinsky one day in late 1995.

Other officials separately escalated attacks on Starr, calling on Attorney General Janet Reno to consider firing the special prosecutor and on Congress to repeal the independent counsel law under which he was first appointed.

But the real action was in the federal courthouse in downtown Washington, where the proceedings remained cloaked in secrecy and lawyers for various participants were anxiously guessing about what Lewis's overtime presence meant for the investigation into the nature of the relationship between Clinton and Lewinsky and any attempts to coach her to lie about it to lawyers in the separate Paula Jones civil case. It was not clear when Lewinsky's appearance would be rescheduled. She remained in Los Angeles at the home of her father, and her lawyer, William H. Ginsburg, told reporters there had been "no negotiations, no contacts" with Starr over a possible cooperation agreement. "He never calls, he never writes," Ginsburg joked.

Ginsburg has said he plans to ask a federal judge to force Starr to give his client full immunity, as the lawyer contends the prosecutor agreed to do. But Starr wants to interview Lewinsky in person before waiving prosecution and may choose to compel her testimony with a grant of limited immunity -- similar to that given her mother -- if she does not cooperate.

If Starr intended to turn up the heat on Lewinsky by calling her mother to testify before the grand jury, he at least apparently succeeded in making Lewis uncomfortable.

Lewis, who was all smiles on Tuesday after two hours of testimony, looked haggard when she arrived at the courthouse yesterday morning. She left the building for lunch but returned appearing angry and upset. About 3:35 p.m., a courthouse nurse equipped with a first-aid kit was summoned to the third floor to tend to Lewis and stayed about 10 minutes. Shortly afterward, Lewis emerged and appeared to be fighting back tears as she left the building.

"She's fine, she's fine," Martin said. "She does promise the press that she will make a statement when she concludes her testimony."

The prosecution team did not look much cheerier when it left the courthouse only a few steps behind Lewis. Prosecutors and investigators alike appeared downcast and worn out from what apparently was a grueling day.

At the White House, Hillary Clinton said the president was holding up fine under the stress. During an invitation-only briefing for selected reporters about planning for events marking the millennium, the first lady said Clinton was preoccupied with the possible impending attack on Iraq, not his troubles with Lewinsky and Starr.

"He is doing very well," Hillary Clinton said. "He is spending a lot of time talking with leaders around the world and consulting with his political, diplomatic and military advisers about the situation in Iraq. That is the primary thing on his mind right now."

The first lady, who has blamed her husband's troubles on a "vast right-wing conspiracy," said the president's poll numbers have shot up since the sex-and-coverup allegations first erupted because "Americans are smart, fair-minded, savvy people" and "see things for what they are."

"We have already seen how much of this charge-and-countercharge does not stand the scrutiny of much attention," she said. "I don't anticipate this will evaporate, but I do anticipate it will slowly dissipate over time by the weight of its own insubstantiality. I am advising everybody to take a deep breath and watch this development. Some of the developments over the last week or two should certainly give everyone pause."

Among developments that the White House sought to dissipate yesterday was the account of retired Secret Service officer Lewis C. Fox, who told The Post that he once was posted in the hallway outside the Oval Office while Lewinsky spent part of a weekend afternoon inside alone with Clinton.

Fox is the first person to say publicly that he saw Clinton spend time alone with Lewinsky. In Clinton's Jan. 17 deposition in the Jones case, the president said he did not recall ever being alone with her, except perhaps for a few moments while transacting some business, according to sources familiar with his testimony.

Fox, who retired a year ago after 27 years with the Secret Service, said Lewinsky arrived with paperwork for Clinton in the fall of 1995 and the president told him to usher her into his office. Fox said that he remained at his post in the hallway for roughly the next 40 minutes while Lewinsky was inside.

Fox could not specify the date of Lewinsky's visit but said he believed it was on a Saturday between September and November 1995. Linda R. Tripp, a colleague of Lewinsky's who secretly tape-recorded their conversations, said in a sworn statement in the Jones case that Lewinsky told her she began having a sexual relationship with the president Nov. 15, 1995.

The White House did not directly deny Fox's story yesterday and aides said they would not ask the president whether it was true or not. Instead, they questioned its plausibility.

White House spokesman Joe Lockhart said Secret Service agents do not typically admit people to see the president. Under normal policy, Lockhart said, uniformed officers such as Fox are stationed down the hall when Clinton is in the Oval Office, while plainclothes agents remain in closer proximity. As a result, Lockhart said, a guard in that location would not be able to see the door through which Lewinsky might have exited.

Other Clinton allies attacked the story as well. Political consultant James Carville said the officer could not have known how long Clinton and Lewinsky were together. "I've been in the Oval Office any number of times and have gone in any number of entrances and have gone out" a different one, he said.

Clinton advisers said that Fox's account to The Post was somewhat different from those he recently gave a local newspaper and television station near his home in Waynesburg, Pa.

In an article in the Observer-Reporter, a newspaper in Washington, Pa., Fox was quoted saying he believed it would be difficult for Clinton and Lewinsky to have had a sexual encounter in the Oval Office because of its windows and because a security officer and a valet usually were nearby. From the Oval Office, the president has access to a private study, where Lewinsky reportedly said on the Tripp tapes that she had sexual encounters with Clinton.

Fox told the Observer-Reporter that during the time he worked in the White House's West Wing, he never saw anything to suggest that the president was having an affair with Lewinsky or anyone else. As he did in his interview with The Post, Fox told local media that he did not know what took place while Lewinsky was with Clinton.

In light of Fox's comments, attorneys from Starr's office met yesterday with Treasury and Justice Department officials to discuss gaining access to Secret Service personnel and records. Even before Fox surfaced, the Secret Service voiced concern that cooperating with Starr might jeopardize its mission of protecting the president by revealing its methods and by breaking the bond of confidentiality between agents and the first family.

An official familiar with the talks said that Starr, who has expressed considerable deference to those concerns, is now on the verge of making a formal request for the first time for information from the Secret Service.

Asked last night about a New York Times report that members of the Secret Service had been subpoenaed, Edward S. Knight, Treasury general counsel, would not comment, saying, "We have a policy that we will not make any comments about the particulars of this investigation."

Separately, Sen. Robert G. Torricelli (D-N.J.) sent a seven-page letter to Reno asking her to investigate Starr's conduct and consider removing him. And two state attorneys general, Republican Grant Woods of Arizona and Democrat L. Scott Harshbarger of Massachusetts, were circulating a letter among their counterparts in other states urging the repeal of the independent counsel law for future matters "to restore accountability." Staff writers Susan Schmidt, Toni Locy, Jacqueline Trescott, Clay Chandler and Roberto Suro contributed to this report. CAPTION: Marcia Lewis, left, leaves court for lunch. At right are Julie Carey of WRC-TV and Lewis's lawyer, Billy Martin. Lewis is set for a third day of testimony today.