Robert E. Dively Jr., a 27-year-old lawyer, capitalized on strong support from abortion opponents last night to win the Republican nomination for a Fairfax County seat in the Virginia House of Delegates being vacated by Democratic Del. Vivian Watts.

A short time later, 39th District Democrats in a separate caucus chose Alan Mayer, a 60-year-old retired Central Intelligence Agency employe, to face Dively.

Mayer said in response to questions that he was prochoice on the abortion issue but expressed the hope that the campaign would not center on a single issue.

Dively and Mayer will run in a Jan. 14 special election prompted by Gov.-elect Gerald L. Baliles' surprise appointment of Watts to be secretary of transportation and public safety in his administration.

Watts, a moderate Democrat, was reelected only two months ago to a third term in the General Assembly.

In the Republican balloting, Dively, who describes himself as a "common sense conservative," easily defeated Louis L. Guy Jr., who had the backing of the GOP hierarchy.

Dively defeated Guy by 351 to 243 in a five-hour party caucus at Jefferson High School. Finishing a distant third with 94 votes was Robert E. (Lee) Murphy, who lost to Watts in the November general election. P.J. Kirk finished last with 39 votes.

Mayer edged party activist Jane King 220 to 199 to win the Democratic nomination. Ernest Lotito finished third with 146 votes and James McConville received 105.

Dively, who is vice president of the Fairfax County Young Republican Club and making his first bid for public office, said he would campaign for improved transportation and quality education. He also pledged to uphold the state's right-to-work law and "the rights of the unborn."

Despite his antiabortion stand and his advocacy of conservative causes, Dively said, "I don't see myself as a New Right person."

Mayer said he plans to campaign on the "mandate" he said that Watts had established over the last four years. "I'll be part of the Northern Virginia team and will do my best to deliver the Northern Virginia agenda," he said.

Mayer claimed to have "strong support from moderate Republicans." He cited his appointment to the Fairfax County Park Authority by Republican Supervisor Thomas M. Davis III as an example.

Questioned about Dively's antiabortion position, Mayer said abortion is "something nobody likes, but it should be left up to the individual."

Watts' decision to step down from the House breathed new life into the sagging fortunes of Annandale-area Republicans, who saw Murphy lose to her by a lopsided 8,274 to 4,530 votes in the November election.

"To organize a campaign in just two weeks is going to be a herculean task, but we're just delighted at having the chance," said Mary Lou Wentzel, vice chairman of the Fairfax County GOP.

Democrats, on the other hand, were clearly dismayed at being forced to try to win the seat all over again, this time without a candidate with name recognition or a sizable power base.

"Obviously Vivian was well established and well known and there was never any serious opposition" to worry about, said Pat Watt, the county Democratic chairman. "We're going to go into this election with a candidate who's relatively unknown and untested, but I'm optimistic that we're going to hang on" to Watts' seat.

Watts' departure from the legislature leaves the House of Delegates with 65 Democrats, 33 Republicans and two independents.

Last night's balloting followed five days of frenzied politicking -- mostly by telephone -- as the hopeful nominees sought to line up support and party officials tried to generate interest in the caucuses.

Those efforts were impeded by the Christmas holidays. Many activists in both parties have been away on vacation, making it difficult for officials to gauge the sentiment of a reliable cross-section of their respective parties.