Conservatives established firm control of the Fairfax County Republican party yesterday as close to 2,000 delegates and supporters gathered to elect former Air Force Brig. Gen. Ben Partin as county chairman.

Partin easily defeated moderate Joan Jacobs after a sometimes bitter campaign to decide who could claim to represent the mainstream of Northern Virginia's growing Republican party. Jacobs supporters claimed that Partin would favor ultraconservative candidates and neglect more moderate Republicans, but Partin pledged to work with everyone in the party.

Last fall's unsuccessful Republican gubernatorial candidate, J. Marshall Coleman, and former attorney general candidate Wyatt Durrette both appeared yesterday, serving to remind the convention at Fairfax High School that fundraising may be more difficult this year. Northern Virginia's two freshmen Republican congressmen, Frank Wolf and Stanford E. Parris, also dropped by to ask support for their reelection campaigns and to deny that they are trying to put distance between themselves and President Reagan.

Parris' likely opponent, former rep. Herbert E. Harris II, also spent the day convention-hopping, as Democrats held mass meetings in eight districts of Fairfax, Prince William and Stafford Counties and in Alexandria to choose delegates to their congressional convention. As expected, Harris appeared to clinch the nomination for a rematch against Parris, who defeated him in 1980.

The Partin-Jacobs fight was the latest in a long war between two Republican factions, both of which call themselves conservative and both of which claim to speak for their party. The Partin wing considers the opposition to be liberal and out of touch with Republicans, while the Jacobs wing calls the opposition ultraconservative and less likely to win a general election.

Despite earlier rumors that the conservative wing would challenge Parris for the congressional nomination, Partin yesterday strongly endorsed both Parris and Wolf. But the well-organized conservative campaign may affect several moderate Republican state delegates expected to face challenges from the right this fall.

Lawrence D. Pratt and John S. Buckley, both former state delegates associated with their party's right wing, were instrumental in orchestrating Partin's victory, and both men live in districts now represented by moderate Republicans, Warren E. Barry and John H. Rust Jr. Buckley, a lobbyist for the National Tax Limitation Committee, said yesterday he has not decided whether to challenge Rust, a Fairfax City lawyer.

Robert E. Harris, another moderate Republican state delegate, said yesterday that Partin represents a narrow base of single-issue Republicans who can defeat other Republicans but not strong Democrats.

"I think Ben would fall in the political spectrum of ultraconservative," Harris said. "He presented his point of view to the general electorate in 1981, and the voters rejected it." Partin ran unsuccessfully for state delegate last year.

"Our side is a coalition of new right and longtime conservatives," Buckley responded. "Some of the people who are touting how they work for all Republicans were actively working against some of us last year."

"We're not radical or far out," said Partin supporter Adrena H. Palmer of Mount Vernon. "We're fundamental, Christian citizens of this country."

Parris, who recently has criticized elements of Reagan's proposed 1983 budget after enthusiastically supporting his economic plan last year, solidly identified himself with the administration before the partisan crowd. "If there just happen to be one or two of you here who think we do not support our president, think again," Parris said. "We have not forgotten that the policies of Jimmy Carter, Herb Harris and other liberal Democrats brought this nation to the brink of fiscal bankruptcy and military disaster."