When Monica Lewinsky's legal troubles began last January, her first phone call was to her mother, Marcia Lewis. The two, by all accounts, are extremely close.

So close, it seems, that when federal prosecutors granted Lewinsky full immunity from prosecution on Tuesday, a similar deal was given Lewis, Lewinsky's shopping partner, roommate and, perhaps most relevant, her confidante.One lawyer familiar with the case said it was the mother's legal vulnerability that ultimately helped bring Monica Lewinsky to the table.

"There is a well-documented case on the mother," one lawyer familiar with the case said Wednesday. "They always knew the pressure point was Monica's mom."

All of the possible evidence against Lewis is not known. But in conversations taped by a former colleague, Linda Tripp, Lewinsky, according to lawyers who have heard the tapes, is quoted as saying that her mother thought it was a "brilliant" idea for Tripp to fake an injury to avoid receiving a subpoena in the Paula Jones sexual misconduct case.

She also says that her mother told her to lie about the nature of her relationship with President Clinton in a deposition to the Jones lawyers. If that is true, Lewis could have been charged with obstructing justice.

Given their strong relationship, Lewinsky is believed to have confided to her mother many of the details of what she now reportedly acknowledges was a sexual relationship with the president.

That knowledge, perhaps imparted over time as any relationship developed, could be crucial to the case that independent counsel Kenneth Starr appears to be building against Clinton.

"She knew everything," Lucianne Goldberg, the New York literary agent who has heard some of the tapes, said Wednesday. The impression given on the tapes, she added, was that the two are "very close."

That became evident from the start of Starr's pursuit of Lewinsky. When federal agents working for Starr first confronted her, she called her mother, and the agents had to wait in a hotel in northern Virginia for hours for Lewis to take the next train from New York.

"What's the big deal?" Lewis asked the investigators, according to several reports published since then. "So she lied and tried to convince someone else to lie."

To Steven Schragis, who published a book that Lewis wrote on the private lives of three male opera stars two years ago, it made perfect sense that her daughter would call her in a time of trouble.

"Marcia Lewis was an extremely competent, aggressive, intelligent, worldly woman," Schragis said. "I'm using a lot of cliches here, but she knew the score. She knew what was going on. The light bulb was on."

In February, Starr called Lewis before a grand jury, and the result was a public relations disaster for the independent counselor. Lewis testified for two days but became ill at one point, leaving the courthouse looking pale and distraught.