NEWS ANALYSIS : ‘Midnight Caller’ Will Be on the Line : Whitewater: GOP senators are eager to learn whether a phone call from Susan Thomases led to any interference in the Vincent Foster suicide probe.

By all accounts, a telephone call from Susan Thomases--one of First Lady Hillary Rodham Clinton’s best friends--has a remarkable way of getting the attention of the President’s top advisers.

Even though Thomases, a high-powered New York lawyer, has the habit of phoning White House aides at their homes late at night, her calls are gratefully accepted.

“In our office she is known as the ‘midnight caller,’ because that’s when she has all her ideas,” observed Margaret Williams, the First Lady’s chief of staff and a frequent recipient of nocturnal calls from Thomases.

When Thomases testifies this week before the Senate Whitewater investigating committee, the question the panel will be trying to answer is whether it was a Thomases “idea” that led White House officials to interfere with the police investigation of the 1993 suicide of Deputy Counsel Vincent Foster.

As a friend and unpaid adviser to Mrs. Clinton, according to White House insiders, Thomases often has served as a go-between for the First Lady in influencing important decisions. In other words, when Thomases calls, White House aides are never quite sure whether they are hearing the recommendations of Mrs. Clinton’s friend or of the First Lady herself.

For that reason, they often heed Thomases’ advice.

That appears to have been the case when Thomases called then-White House Counsel Bernard Nussbaum on July 21, 1993, to discuss the procedures that would be used to allow law enforcement officials to inspect Foster’s office after his death. Republican critics are suggesting that Thomases’ recommendation to bar police from the office represented an attempt by the First Lady to prevent potentially embarrassing private information from coming to light, perhaps involving the Clintons’ Whitewater investment.

“Susan Thomases was deeply involved in trying to affect the outcome of the discussion about what to do with the files” in Foster’s office, insisted Sen. Connie Mack (R-Fla.), a member of the Whitewater panel.

Her explanation of the call is being eagerly awaited. What intrigues Republicans about Thomases is not only her close relationship to Mrs. Clinton but her reputation as a firebrand and an unabashed liberal who has been involved in some of the Clinton Administration’s bigger disasters. For example, she is said to have put forth the names of both Zoe Baird and Kimba M. Wood for attorney general as Clinton sought to form his Cabinet. Both names were withdrawn when controversy developed over the women’s employment of illegal immigrants as domestic help.

Currently a partner in the blue chip New York law firm of Wilkie, Farr & Gallagher, Thomases cut her political teeth in the 1968 Democratic insurgent presidential campaign of Sen. Eugene J. McCarthy. She also worked in political campaigns for former Vice President Walter F. Mondale and Sens. Edward M. Kennedy of Massachusetts and Bill Bradley of New Jersey before acting as scheduler for Clinton in 1992.

Although she turned down a job in the Clinton White House, she is a frequent presence in the West Wing offices. And even though the President’s aides claim to honor her advice, they clearly do not always enjoy it.

Evelyn Lieberman, currently a White House press aide who previously worked in the First Lady’s office, wore a painful expression on her face as she described for the committee the many messages that Thomases has left on her home answering machine long after normal business hours.

In fact, it appears that White House officials are depending on Thomases’ reputation as a self-important meddler to explain away the 13 telephone calls she made to top White House aides from 12:15 a.m. July 21, 1993--hours after Foster’s body was found--to 5:30 p.m. July 22.

“Susan Thomases is not just anybody,” insisted committee Chairman Alfonse M. D’Amato (R-N.Y.), who, like Mack, sees the calls as evidence that she was dictating to the White House how to respond to the suicide inquiry. “She does not call on whim.”

In response, Williams in her testimony tried to belittle Thomases’ role as an important adviser. She said that when Thomases visited her office in the White House in late July, 1993, Mrs. Clinton’s friend seemed most preoccupied with the question of whether Foster’s family would be able to collect on his life insurance.

“I would encourage you not to be so certain that there was something sinister here,” Williams told the Republicans who are suspicious of Thomases’ role. “This list [of 13 phone calls] does not suggest to me what it suggests to you.”

Among the 13 calls, the one that most interests Republicans is Thomases’ long-distance discussion of the suicide investigation with Nussbaum. After Nussbaum talked to Thomases, Stephen Neuwirth, a White House attorney, said he had the distinct impression that both Thomases and the First Lady wanted to restrict the access of law enforcement officials to Foster’s office.

At the time he took his life, Foster was working on a number of personal legal issues for the First Family. Based on Neuwirth’s account, Republicans have advanced the theory that Thomases and Mrs. Clinton sought to restrict access to the office because the First Lady feared that law enforcement officials would find some embarrassing secret about the Clintons’ finances.

In depositions, Nussbaum and Thomases offered slightly different versions of what was said in their phone conversation. Thomases said that Nussbaum outlined how he intended to handle the investigation. Nussbaum recalled that it was more of a give-and-take discussion. Neither of them said that they remembered any reference to the First Lady, although they do not deny that her name might have come up in the conversation.

When Thomases is called to the witness table Tuesday, she is expected to shed some light on how Mrs. Clinton reacted to Foster’s death. Her appearance will also offer a rare close-up of an influential, behind-the-scenes player in the Clinton Administration.