A political brawl erupted yesterday that will determine who controls Fairfax County's badly divided Republican Party -- a retired champion seeking a comeback or a younger man with a mastery of modern technique who now serves as party chairman.

A boisterous band of party rebels braved the morning chill outside the front door of county GOP headquarters in Annandale to hear former county sheriff James D. Swinson, 73, announce his candidacy for chairman of the Republican Party in Virginia's largest and most affluent locality.

The announcement by Swinson, a self-described "tough old goat" who was one of the few elected Republicans in Fairfax during most of his 16 years as sheriff, amounts to a revolt by the party's elected Republican officeholders against the arch-conservative party leadership.

All but three elected Republicans from Fairfax have endorsed Swinson, and the ones who have not are remaining neutral. The outcome will be decided at party mass meetings tentatively scheduled for March 11.

Some GOP officeholders in the county charge that the current chairman, Benton K. Partin, has shut them out of key decisions, imposed tests of ideological purity and repeatedly broken the celebrated 11th Commandment: Thou shalt speak no evil in public of a fellow Republican.

Partin, 60, who has led the severely divided party since 1982, was not at yesterday's rally, although his name was on everyone's lips. He has said he plans to seek an unprecedented third term, however, and he declared that electing Swinson, who left office in 1980 under fire for his management of the county jail, would be "an embarrassment."

Swinson, a jovial man who was once renowned for labeling his political opponents "knuckleheads," had milder words yesterday for Partin.

"I'm not mad at anybody and I don't have the ax out for anybody," he said. "What I want to do is bring the party together."

Without naming Partin, Swinson said the party leadership "has taken the position that 'if you don't believe as I do, then you're out.' A political party that restricts itself to such a narrow base cuts its own throat," he declared.

In an interview Thursday, Partin said the split in the county's Republican Party is between the "Reagan crowd," which he said he represents, and the "so-called moderates who don't have a platform and don't support Reagan."

"They call themselves moderate, but what are they?" said Partin. "Somebody said a moderate's a liberal running for public office." He blamed the moderates for the defeat of Republican gubernatorial candidate Wyatt B. Durrette in Fairfax last fall and charged that they had "boycotted" the party for the last four years.

Two of the key Republican officeholders behind Swinson's candidacy, Supervisor Thomas M. Davis III of Mason District and Supervisor Nancy K. Falck of Dranesville, were not at the rally yesterday. They have both spoken harshly of Partin's leadership in the past and were instrumental in persuading Swinson to challenge Partin.

"This will be expensive, it'll be well organized on both sides, it'll be a real test of strength," Davis said in an interview Thursday. Davis called himself "a conservative who wants a broad-based party." Swinson said he expected his race to cost $20,000.

Although Partin has an array of political problems to overcome, Swinson is also saddled with considerable -- if somewhat dated -- political baggage.

In the second half of 1978, three prisoners in the Fairfax County jail, all of them black, died while under Swinson's custody -- a fact that outraged the sheriff's mostly Democratic critics and brought calls for his resignation.

Swinson further enraged his detractors when, in his defense, he told a reporter: "Going through the door of my jail is not a ticket to eternal life. People have died there before and people will die there again. It doesn't make a . . . if I'm sheriff or if you're a sheriff."

After an investigation and two lawsuits alleging wrongful death, the county sheriff's office, in out-of-court settlements, agreed to pay damages to the families of two of the dead inmates.

Swinson's hand-picked Republican protege, Wayne M. Huggins, was elected as Swinson's successor in 1979 and took office the next year. Huggins introduced Swinson at the rally yesterday, calling him "one of my closest friends and political allies."