San Francisco Mayor Dianne Feinstein appears about to win a bizarre recall challenge with the largest turnout of absentee voters ever in her city, one more sign of the growing potency of mail ballots in American politics.

Forced into Tuesday's recall election by 24,000 signatures collected by a communist anti-gun-control group called the White Panthers, Feinstein has launched a $400,000 campaign so successful that it may leave her without any major opponent when she runs for a second four-year term in November.

A recent San Francisco Examiner poll shows 75 percent of voters opposed to her recall. Her campaign volunteers have successfully solicited 50,000 absentee votes in an election unlikely to draw more than 140,000 voters.

Combined with her success in luring the 1984 Democratic National Convention to San Francisco, Feinstein's strong response to the recall challenge has made her far more formidable politically than three months ago.

At that time, disenchantment among gays and liberals with her efforts to steer a middle course through the city's political mine field had led several local politicians to consider running against her.

"She is so much stronger than she was before," said Wendy Nelder, president of the Board of Supervisors, who opposes Feinstein's recall but has been an adversary of the mayor on some issues.

The White Panthers attacked Feinstein for signing gun control ordinances that would keep them from arming themselves against police harassment. This forced "generally responsible San Francisco people who disagree with her to come to her defense," Nelder said.

In an interview, Feinstein acknowledged that, by forcing her to build a campaign organization so early, the recall may have frightened off potential November opponents.

Even gay activists, who bitterly protested her veto of a bill to give homosexual partners the equivalent of a city marriage license, have been unable to develop the initial anti-Feinstein resentment that fueled the White Panthers' recall drive.

The Examiner poll of 503 likely voters showed 75 percent against recall, 14 percent in favor and 11 percent undecided. Among the 18 percent of those polled who said they were gay, 71 percent opposed recall, with 25 percent in favor and 4 percent undecided.

"The outlook looks bleak," said Carole Migden, president of the Harvey Milk Gay Democratic Club. Most gay leaders rejected the White Panther recall petitions but, when the recall election was scheduled, they decided to present their case against Feinstein.

The Harvey Milk club was named after the gay city supervisor murdered with Mayor George Moscone by anti-gay supervisor Dan White in 1978. Its leaders have charged that Feinstein, Moscone's successor, failed to honor campaign promises to appoint more gays to city posts and end harassment of homosexuals by police and gangs.

Migden said she hopes for "at least 25 percent for recalling Feinstein, or maybe 30 percent. Anything beyond that is a victory."

According to Feinstein's deputy campaign manager, Fred Ross, Feinstein suggested the ballot-by-mail campaign. Pollster Mervin Field had pointed out to her that Republican George Deukmejian won the California governship on the strength of a $350,000 GOP mail-ballot campaign.

The party mailed absentee ballot applications to Republicans, and party workers took the 100,000 completed applications to voting registrars. The Feinstein campaign spent only $40,000 to stimulate 50,000 absentee ballots by forsaking the mail and setting up ironing boards in shopping centers with stacks of absentee ballot applications on them.

Volunteers told recall opponents passing by that it would be much more convenient to vote in a special, one-issue election by mail. California law no longer requires absentee voters to certify that they will be absent on election day.

City election officials have said they expect 70,000 absentee ballots Tuesday, compared with 31,000 in the 1980 presidential election.

Superior Court Judge Stuart Pollak, who threw out a White Panther challenge to the mail ballot campaign, said before overruling the argument that the system compromised ballot secrecy that he had voted absentee.