Two years ago when Rep. Elizabeth Holtzman (D-N.Y.) told Pamela Harrisman over lunch that she thought she'd run for the U.S. Senate in 1980, Harriman urged her not to.

"I said, 'Please don't as long as Jacob Javits is still running,'" Pam Harriman remembered telling Holtzman, adding last night, "which just goes to show you how dumb I am."

Not that Harriman or anybody else could have foreseen the outcome of last month's primaries: Holtzman is in as New York's Democratic nominee for the Senate and Javits, the incumbent for 24 years, is out as the Republican nominee.

And faces Liz Holtzman once might not hve expected to find at a fund-raiser for her stood out in the crowd last night at the Harrimans' Georgetown home. Besides representatives of big business like Philip Morris, Warner Communications and Occidental Petroleum, there were leaders from 15 trade unions, among them those that deal with garment, marine, transportation and machine industries.

"It shows a different trend, since they'd been with Javits until the primary," said Janet Howard, an aide to Pamela Harriman.

It also showed just how far a little success can go. With a whole new business-labor support system building on her behalf, here was Liz Holtzman hobnobbing with some of the biggest guns in the Carter administration.

The biggest was Vice President Walter Mondale, who assured the crowd that "there isn't a better candidate running for the U.S. Senate anywhere in this country." He first marked Holtzman as a "comer in American politics," he said, when he watched her on the House Judiciary Committee during the impeachment hearings of Richard Nixon.

Besides Mondale, there was Robert Strauss, chairman of the Carter-Mondale campaign, bringing tidings of joy about Texas ("a CBS poll says we're leading"), Secretary of the Treasury G. William Miller, Energy Secretary Charles W. Duncan Jr. and Sen. Paul Sarbanes (D-MD.).

Heady stuff for a candidate who went into the primary without the support of even New York's big Democratic guns.

"One of Liz's people said to me just now," Pamela Harriman said last night, "that it's so nice to be part of The Establishment for a change."

If it was nice for Holtzman, there was the oh-so-subtle suggestion that bringing her into The Establishment also might be nice for Jimmy Carter, who desperately needs New York's 41 electoral votes.

"I can help reelect President Carter and Vice President Mondale," she told supporters who paid $500 each for a total estimated at between $65,000 and $70,000.

"Everybody thinks she's going to win, even with Javits staying in [on the Libral Party ticket]," said Evelyn Dubrow of the International Ladies Garment Workers. "We'd like it better if Javits would recognize that he can't win and that he's only taking away from her. It's not a desertion, the way he thinks it is -- after all, everybody has to know when his time comes -- but if we don't elect Liz, we're going to get [GOP nominee Alfonse] D'Amato."

But that prospect seemed remote to last night's crowd as Strauss stood on the spacious porch of the Harrimans' elegant house and reminded everyone of the successful campaigns launched there in past years. Hugh Carey's race for governor of New York got a boost there, also Jimmy Carter's campaign for president in 1976.

"Things have a way of happening in threes," said Strauss.