SAN FRANCISCO (AP) _ Wealthy Democratic fund-raiser Nancy Pelosi, supported by a solid but indecisive victory in a special election, is favored to win Tuesday's runoff for the 5th District seat vacated by the death of Rep. Sala Burton.

''Pelosi Wins,'' headlines read in April after she drew 36 percent with 38,021 of the ballots in a field of 14, in the district which has had a Democratic majority for the past 50 years.

But because she didn't get half of the votes, she was forced into the runoff with a Republican and four independents, none of whom got more than 6 percent in the April election.

The Republican in the runoff is Harriet Ross, a city deputy public defender and former San Francisco Republican Party chair. She got 2.8 percent or 2,922 votes in April.

The independents are Ted Zuur of the Peace and Freedom Party, Cathy Sedwick of the Socialist Workers Party, Karen Edwards of the Humanist Party and Samuel Grove of the Libertarian Party.

Pelosi's nearest competitor in the April election was San Francisco Supervisor Harry Britt, a homosexual rights activist who counted on broad support from the gay and straight communities. He posted 32.2 percent.

Pelosi has never before held elective office, but has spent 18 years as a Democratic fund-raiser and activist, including chairwoman of the 1984 Democratic Party convention host committee and high positions in the state and national party.

The 5th district includes about 75 percent of San Francisco, excluding the northwestern portion of the city and the area west of an irregular line from the U.S. 6th Army Presidio south to Market street.

Its voters had sent Democrat Phillip Burton to Congress for two decades, then passed the mantle to his wife upon his death. Before her death from cancer, Mrs. Burton announced Pelosi was her choice for a successor.

Pelosi got a strong whiff of political hard ball during the campaign before the special election when Britt, in an effort to prove his support went beyond the gay community, disclosed he had hired a private detective to investigate Pelosi.

''When you're running against an opponent with no record, you have to do a little research to find out where she stands on things,'' said Britt.

But it drove Pelosi close to tears when she spoke afterwards about the probe. ''I don't have anything to hide, so I'm not worried about it,'' said the mother of five children.

Pelosi's campaign is based on the theme that, better than her opponents, she knows the Capitol and the powerful people who walk its corridors through her work with the Democratic Party.