Fairfax County Board Chairman John F. Herrity, collecting chips for the biggest political poker game of his life, raised more than $70,000 yesterday in what he called "the largest fund-raising gathering for a locally elected official . . . in the history of Fairfax County."

About 400 Republicans, primarily the county's business elite, paid at least $100 for breakfast at the McLean Hilton to hear Sens. Robert J. Dole (R-Kan.), Paul S. Trible (R-Va.) and a phalanx of local GOP officials extol Herrity's leadership in 12 years as Fairfax's top elected official. An undisclosed number of "sponsors" paid as much as $500, Herrity campaign officials said.

Herrity called it "a great outpouring of money and affection" and raised more in this single event than he did in his entire 1983 reelection campaign, and possibly as much as he did in his 1983 and 1979 races combined.

In a brief address, Dole said, "I know for a fact what an outstanding job Jack Herrity has done . . . . You've already given your money so it's not necessary that I say anything good about Jack Herrity. The money's being counted now."

Behind the veneer of good cheer and applause, however, was concern on the part of many Republicans who think Herrity's opponent, Democratic Supervisor Audrey Moore of Annandale, may hold a substantial lead with less than six weeks to go to the Nov. 3 election.

One organizer from Herrity's home district of Springfield said he was having a hard time finding volunteers. A high-ranking party official acknowledged he was worried about Herrity's chances. One GOP officeholder said bluntly: "Democrats are winning the grass roots." Another officeholder said polls in his generally Republican district look bad for Herrity. Even in Fairfax County areas that have traditionally backed Herrity strongly, several Republicans said privately, Herrity is running even with Moore.

Some Republicans, while acknowledging that Herrity may be trailing, insisted that he will close the gap by Election Day. Most predicted a media blitz in the coming weeks that would try to whittle away at Moore's support by attacking her record as an opponent of several major road projects.

Moore campaign spokesman Janice Spector dismissed Herrity's fund-raiser. "That he has X number of people who can write him a substantial check doesn't mean he's going to win," she said.

None of that seemed to faze Herrity. In an energetic rendition of his basic speech, he belittled the slow-growth policies pursued by the county in the early 1970s -- policies he suggested Moore would resurrect. Efforts by the county board of supervisors of that era to restrict development, he declared, were a "total failure," driving up real estate taxes and squashing proposed new roads before state courts and lawmakers put a stop to the policies.

With his election as chairman in 1975, Herrity said, Fairfax entered a new era of cooperation between business, government and citizens. "All came together to create the finest place to live and work in this country today," he said.

Although the reception to his speech was generally restrained, Herrity drew loud applause when he hailed Fairfax's economy as one in which "jobs go out looking for people, not people looking for jobs."

He also attacked Moore as "someone who's tried to kill every major road project" in the county. Declaring that it is "utter nonsense" to try to stop growth, he added: "You don't go to a mortician to get an appendectomy. You go to a doctor to get an appendectomy and you go to Jack Herrity's leadership to get those roads."

Herrity has declined to say how much money his campaign has raised since mid-August, when campaign officials reported that they had raised about $200,000. Since then, Herrity's fund raising is believed to have picked up steam, and several Republicans speculated that the total may now be approaching $400,000.

In his 1983 race, Herrity raised about $55,000; in 1979, he raised about $34,000.

Moore, despite having nearly twice as many donors as Herrity, has not raised as much money. Her campaign spokesman said yesterday that Moore's total is "approaching $200,000."

Among those at the fund-raiser were several real estate developers. Herrity has pledged not to accept campaign contributions from developers, on whose land-use applications he must vote. One source said the developers had not been asked to pay the $100-a-plate fee and had been invited as a courtesy to Dole, who wanted to meet some of Northern Virginia's major contributors.

Although Herrity is not accepting donations directly from developers, he is relying on them to do a substantial amount of his fund raising. In a meeting this summer with Herrity, several of Northern Virginia's most prominent business executives, including developers, agreed to raise $10,000 each on Herrity's behalf from their colleagues and associates.