Karen and Bill Speros' four-bedroom, $450,000 home looks out over Orange County's grass-covered Pacific Coast hills, the abode of affluent, conservative Republicans wedded to the idea of a strong national defense.

Yet among the Speroses and their friends, the talk has suddenly turned to how quickly their paneled walls and landscaped gardens could disintegrate in a nuclear war.

"If you've got to gamble," said Bill Speros, one of many conservative Reagan supporters here who differ with the president on the arms race, "it's better to gamble on a nuclear freeze than on an all-out nuclear buildup."

In this huge, traditionally Republican southern California county of engineers, business executives, lawyers and teachers, where presidential candidate Ronald Reagan won 75 percent of the vote, the campaign to freeze production of nuclear weapons has struck a surprising chord. More than 102,000 of the 650,000 California signatures supporting a statewide vote for a nuclear weapons freeze have been collected in Orange County.

At the St. Mark Presbyterian Church in Newport Beach, where the Speroses worship, the largely Republican, pro-Reagan congregation has discussed the petition drive in after-church meetings, and the associate pastor estimates at least half of them have signed.

"I was a little reluctant to embrace the idea because, oh, I guess I didn't want to be associated with a peacenik-type thing," said John R. Fassett, 48, a white-haired, bespectacled engineering manager at one of the high-technology companies that dot this area.

But, he said, he was turned around by the realization that "we have already so many weapons in the nuclear arsenal, even if a small fraction were to be used, the world would cease to be habitable for man."

"Never in our wildest dreams did we think we would have enough signatures already," said Karen Speros, a high school teacher who circulated the nuclear-freeze initiative petitions at county shopping centers.

Jo Seidita, however, who started the initiative campaign with her husband Nicholas, said she had been confident this would happen ever since her husband found a small news article about the votes on a 1980 nuclear freeze proposal in a relatively conservative East Coast area--three state senatorial districts that are "sort of the Orange County of Massachusetts."

The local Massachusetts resolution carried 30 of the 32 towns where Reagan had also received a majority. The Seiditas persuaded their small Unitarian church in Los Angeles' San Fernando Valley to help sponsor the initiative campaign, which has now apparently guaranteed that the nuclear freeze proposal will be on the state ballot in November.

At the 425-member St. Mark Presbyterian Church, the 34-year-old associate pastor, Tony Wolfe, has found a receptive audience for sermons decrying the nuclear arms race.

"The court of last resort is in the hearts of Christian people throughout the country who are beginning to question the heretofore sacred domain of military planners, whose only understanding of security is based on weapons," Wolfe told the congregation last August as the freeze campaign was taking shape.

President Reagan and Secretary of State Alexander M. Haig Jr. oppose freezing American and Soviet nuclear weapons production because they feel only the threat of a U.S. arms buildup can convince the Soviets to negotiate an arms reduction on both sides.

But conservative Republicans here say they see the California freeze proposal to be so qualified that it is almost innocuous. It directs the governor of California to ask the president to negotiate a bilateral, verifiable freeze with the Soviet Union.

Karen Speros remembered her husband finally saying, "It's really not very much at all, is it? It's really a public relations tool. It says I would like Jerry to tell Ronnie thus and so."

Bill Speros, 46, is president of a local heating and cooling system construction firm, and the severe slump in the construction market, brought on in part by high interest rates stimulated by government deficit spending, has influenced the family's thinking. "The big bang for the buck has its limits," Karen Speros said.

A January California poll by Mervin Field said 60 percent of California voters favored the nuclear freeze initiative, with 32 percent opposed and 8 percent undecided. Self-described conservatives supported the idea 51 to 41 percent and self-described Republicans were split, 46 percent in favor and 47 percent opposed.

Although both Wolfe and Karen Speros, among the leaders of the nuclear freeze movement here, have been in contact with a militantly antinuclear peace group, the Alliance for Survival, Wolfe said he had tried "to avoid altogether raising the name of the alliance, because that would raise a yellow flag for some of our members."