Alan E. Mayer, an amiable, chatty man of average height and medium build, strikes people as Mr. Everybody: An ordinary fellow from down the block, who happens to be running for office but can't quite believe it himself.

The image is an apt one for Mayer, the Democratic candidate in tomorrow's special election for the 39th District of the Virginia House of Delegates who said that until a few weeks ago he had no plans to turn his experience as a Fairfax County community activist into a springboard for elective politics.

"I'm new at this," the 60-year-old Mayer said, almost apologetically, in a recent interview. " . . . I stopped being a neighborhood person and started being a party person."

Indeed, Mayer said, the first question his Lincolnia neighbors asked when he announced his candidacy was whether he would continue to cut the grass around the neighborhood swimming pool if he becomes a delegate representing the Annandale area. The answer, he promised, is yes.

But Mayer's soft-edged personality combined with a politically moderate platform may actually be a liability in tomorrow's special election.

The reason is that Mayer's opponent, Republican Robert E. Dively Jr., won his nomination largely on the strength of a single issue: his stance against abortion. Because voter turnout is traditionally low in special elections, some Democrats are worried that antiabortion activists will support Dively in force while Mayer's constituency of moderates stays home.

Dively "came from a block group. I came from a grass-roots effort of people in my neighborhood," said Mayer, who describes himself as "prochoice." "The danger is that a determined, well-organized minority could steal the election."

Because the campaign season for a special election is so short, said Fairfax Supervisor Audrey Moore (D-Annandale), many voters are only vaguely aware of the race, which was kicked off when Gov. Gerald L. Baliles tapped Democratic Del. Vivian E. Watts to serve in his cabinet.

"I think that people are aware of it the race but I think they don't feel that they know enough about it to vote," said Moore, a Mayer supporter.

Mayer retired in 1980 after nearly 30 years at the Central Intelligence Agency to begin "the second half of my life." He decided to immerse himself in community affairs and received a degree in urban planning at the University of Virginia, writing his thesis on Fairfax County.

His baptism as an activist came while he was head of a citizens group that sued a developer to prevent him from building a commercial development that the group thought was too dense. The dispute was settled out of court.

Moore said she was impressed with Mayer's persistence in winning her support during the controversy. "To put it bluntly, he bugged me," said Moore. "He bugged and he bugged and he bugged. I'm impressed by that . . . . He's low-key, but don't let him kid you, he's determined."

After that dispute several years ago, Mayer accepted numerous appointments and committee assignments, including a spot on the Fairfax County Park Authority.

Mayer said his stance on many issues differs little from Dively's. He is stressing strong support for transportation and education, virtual prerequisites on the agenda of any Northern Virginia politician.

Mayer, like Dively, supports a $750 million state bond issue to finance road construction.

On abortion, the most prominent issue that has separated the candidates, Mayer said: "Nobody likes abortion. But no parent who has ever raised a daughter hasn't thought to himself, 'What if?' It's a very personal issue . . . and it's not one in which government should intrude."

The candidates also split on the question of whether increased taxes will be necessary to support their spending priorities. Mayer said he would vote for a gasoline tax increase, something Dively has opposed, and he did not rule out a state income tax increase.

"If the choice is between getting these services we need and increasing taxes or not having a tax increase, we'll have to find the revenue necessary."

Mayer estimated that he will spend about $20,000 in his race, roughly the same amount as Dively. About half of that amount came from Watts' campaign organization.

Mayer's race has taken on a special significance because it will be the first test of Democratic strength in the area since the Democratic sweep at the state level last November, Commonwealth's Attorney Robert F. Horan Jr. said.

At a news conference for Mayer last week, Fairfax board Vice Chairman Democrat Martha V. Pennino said that Mayer's support from the Democratic establishment underscores his moderate appeal, in contrast to Dively, who in his primary bid was not the choice of most Fairfax GOP leaders. "Frankly, they all ended up with egg on their face," Pennino said.

Moore said the efforts of Democratic leaders on Mayer's behalf are motivated mostly by a fear of losing the seat. "I think the Democrats realize they're not the favored party anymore, and they have to work very hard to get their candidates elected."