With substantial help from financial backers of former president Ford, a coalition of Republicans opposed to Ronald Reagan appears to have taken a big step toward abolition of California's winner-take-all Republican primary.

If a ballot initiative is adopted in the June 3 primary election, the present system would be replaced by proportional representation. The prospect of this change makes it worthwhile for GOP presidential rivals of Reagan to campaign in California.Republican politicians here believe that adoption of the initiative, which even Reagan aides concede is likely, could cost Reagan at least 80 delegates to the Republican National Convention.

"This initiative unstacks a stacked deck," said state Sen. John G. Schmitz of Orange County, co-chairman of the Fair Presidential Primary Committee.

But Schmitz emphasized that there is still uncertainty about whether the initiative has qualified for the June 3 primary ballot, when its results could affect the distribution of delegates.

Under California law, only initiatives turned in by Nov. 30 are guranteed a place on the primary ballot. If county registrars of voters and the secretary of state verify the signatures on the "fair primary" initiative by Jan. 24, it will be on the June ballot. Otherwise, it will be on the November ballot and would not apply until 1984.

With a well-financed, last-minute signature drive at the Rose Bowl parade in Pasadena on New Year's Day, initiative sponsorws reached a total of 494,217 signatures, with 346,119 needed to qualify.

The cost was more than $125 a signature in the $625,000 campaign, in which $200,000 was loaned and another $25,000 contributed by Leonard Firestone, a leading financial backer of former presidents Nixon and Ford. Other prominent GOP financiers who gave $25,000 or more including Clement Stone, Henry Salvatori, Robert Fluor, Ben Biaggini and David Packard.

If the measure qualifies, its passage seems virtually assured. Every public opinion survey taken in California has for several years shown a large majority in favor of proportional allocation of delegates.

The winner-take-all California primary is the last vestige in the nation of the "old politics" in which a powerful governor or other dominant political figure could determine how a state's delegate votes would be allocated.

In Reagan's case, since all other states have abandoned statewide winner-take-all primaries, the effect of the California sweep would be magnified. The state has 168 delegates to the national convention in Detroit, nearly 17 percent of the number needed for presidential nomination.

Each candidate files a slate of delegates in the June primary. While former governor Reagan would be a prohibitive favorite to win any GOP primary in California, his strategists have assumed that he would get no more than 50 to 60 percent of the vote in a multi-candidate race. If the ballot initiative were approved, the delegates to the national convention would be apportioned on the basis of the primary vote that day.

The initiative was launched after a Reagan-controlled state republican convention last September rejected a modified proportional-representation plan. Heading the campaign were Schmitz, generally considered California's most conservative office-holder, and Rep. Paul N. (Pete) McCloskey, the state's most liberal Republican congressman.

Schmitz is a supporter of John Connally for president and McCloskey a backer of George Bush.

For lack of money the signature campaign lagged badly until late November, when Firestone began taking an active financial role at Ford's request. Ford himself collected more than 100 signatures.

The initiative technically applies to both parties but is meaningless to the Democrats, whose national committee has outlawed winner-take-all primaries. However, Democratic voters as well as Republicans will vote on the ballot measure, further improving its chances.