Robert E. Dively Jr. recently caught the eye of his fellow Republicans with a one-page campaign leaflet during a hastily called GOP caucus to nominate a candidate for a Fairfax County seat in the Virginia House of Delegates.

Three other Republicans were running for the 39th District seat left vacant when Gov. Gerald L. Baliles appointed Democratic Del. Vivian E. Watts to his cabinet. They displayed impressive lists of endorsements and discussed traditional Republican themes such as fiscal conservatism.

But the short statement by Dively, a 27-year-old lawyer and newcomer to elective politics, went a step further: "I support . . . individual rights of citizens and believe in the rights of the unborn," Dively's statement said in part. "I am a pro-life candidate."

When 727 Republicans finished voting, Dively was an easy winner, turning back three older, better-known candidates, including the choice of local and state GOP elected officials in Northern Virginia, Louis L. Guy Jr., by a vote of 351 to 243. Many attributed his victory to that one sentence in his campaign literature.

But although his appeal to abortion foes helped Dively win the nomination, fellow Republicans said privately they were worried that a single-issue candidacy would mean certain defeat during the general election campaign. Since the caucus, Dively has moved to broaden his message.

The young Fairfax County lawyer has stressed transportation, education, taxes and crime as well as the abortion issue in his two-week race against Democrat Alan E. Mayer, 60, a retired Central Intelligence Agency employe. Voters will go to the polls tomorrow night.

It will be the second time in two months that voters of the 39th District, the Annandale area, have chosen a representive for the Virginia General Assembly. On Nov. 5 they elected Democrat Watts, a popular two-term incumbent, who since has been named the state's secretary of transportation and public safety.

Dively has objected from the outset to the contention that his antiabortion stand was responsible for his nomination.

"It's a fundamental issue and I think people should be made aware of it," he said. "But I think I won the election because I've been a team player in Republican politics for a long time."

In a recent mailing to some 39th District voters, Dively referred to abortion as "an issue that has divided our times and rises above all of us. It is the question of who shall live. Right now, your Virginia tax money and mine is used to pay for abortions. I think that is wrong . . . . To be silent or to turn aside an issue of such importance should not be tolerated."

He said that "is exactly what my opponent has done" when Mayer said on the night of his nomination that the question of whether to have an abortion should be left to the individual.

Dively and Mayer have raced to put together vigorous campaigns in the two weeks they were allotted, afraid that many voters may have missed the news of Watts' resignation over the holidays and are unaware of tomorrow's election.

"A lot of people don't even know there's a campaign," Dively said. "There is definitely a lack of interest. Hopefully we're sparking some."

In his effort to attract support, Dively, who is single and lives in North Springfield, has emphasized his record of involvement in GOP politics and local civic organizations. He is vice president of the Fairfax County Young Republican Club and was a coordinator of the unsuccessful 1985 gubernatorial campaign of Rep. Stan Parris (R-Va.).

He is vice president of the Fairfax County Jaycees, and he belongs to the Fairfax County Chamber of Commerce, the Coalition for Balanced Transportation and the Legislative Committee of the Fairfax Bar Association.

Dively has expressed support for Parris' recent call for a $750 million bond issue to finance state road improvements and the retention of the $21 million annual funding for the Metro bus and rail system. He has urged increased spending for education, citing a need for additional student housing at George Mason University.

He said the need for additional services in Northern Virginia can be met without a tax increase, opposing in particular the call by some legislators for a gasoline tax increase.

Dively's efforts were applauded by Mary Lou Wentzel, vice chairwoman of the Fairfax County Republican Party, who said the candidate has run an energetic campaign that has won the support of young Republicans as well as the elected officials who backed Guy for the nomination.

According to Wentzel, Dively is campaigning on a budget of about $20,000. Among the contributors, she said, have been the Republican National Committee, the Virginia State Republican Committee and the Fairfax County GOP.