Never before has so much money been spent so early to win the California governorship--the political plum that has provided nourishment to the careers of half a dozen presidential contenders, including the one now in the White House.

Nor, as candidates prepare for an election still more than a year away, has a more unusual mix of aspirants found its way to the top of the opinion polls, all eager for the job that Gov. Edmund G. (Jerry) Brown Jr. has made a focus of political surprises.

The candidate leading the polls is Tom Bradley, 63, the three-term Los Angeles mayor who could become the first elected black governor in U.S. history. But Bradley has little announced competition in the Democratic primary, so attention has shifted to the increasingly close and expensive contest for the Republican nomination.

For the past two years Republican Lt. Gov. Mike Curb has been soliciting the support of every conceivable Republican officeholder and contributor in order to frighten off any competition. He is a solid Reagan loyalist, with enough support from the president's wealthy friends here to budget a record $3.5 million for the GOP primary, far beyond the $2 million spent by Republican Norton Simon in a 1970 California Senate race, the state's previous primary election spending record.

However, Curb's fund-raising has failed to impress Republican Attorney General George Deukmejian, 53, who agrees with Curb on nearly all issues but senses lingering doubts in the electorate about age, experience and background.

"I believe the voters want stable and experienced leadership," Deukmejian said in an interview at his office here. He mentioned his 16 years in the state legislature and four years as attorney general. Deukmejian expects to spend $2.5 million on the primary. And to combat Curb endorsements from Reagan confidants like businessman Holmes Tuttle, he recently announced the endorsement of Reagan's brother Neil. The president has remained neutral.

At 36, Curb appears to be even younger. He has profited and suffered politically from his early success as a record producer dealing with acts like the Osmond and Boone families who echoed his own clean-cut, avowedly Christian image. Curb dropped out of San Fernando Valley State College to start his own record company and eventually amassed a multimillion-dollar fortune that allowed him to try politics.

Despite his demonstrated business acumen, Curb had never held public office before being elected lieutenant governor in 1978. That lack of experience still haunts him, and gives Deukmejian an opening. "I think people have begun to feel more comfortable with me in office, yet it takes time," Curb said in an interview at his Trousdale estates home here.

Curb again played into Deukmejian's hands this month when his usually sunny demeanor evaporated publicly in a confrontation with a reporter for the Los Angeles Herald Examiner, Linda Breakstone. Breakstone had written an article--published on an inside page--pointing out that several people on a Curb supporters' list did not actually back his candidacy. Although the accuracy of the report was never questioned, Curb lost his temper.

"I'm not going to take this sitting down," he told Breakstone. "I've said that before, but this time I mean it. I'm going to get you."

The remark drew attention to Breakstone's article everywhere, including the Los Angeles Times, which has four times as many readers. Although Curb later apologized, his aides allowed the issue to rise again by bumping Breakstone off a press plane following Curb around the state the day he formally announced his candidacy. At a news conference, Deukmejian questioned Curb's "maturity" in handling the situation.

If it were not for the difference in their ages and backgrounds, Curb and Deukmejian would seem like political twins. Both grasp the president's coattails firmly: Curb mentioned in his campaign opening speech his role as "national vice chairman of President Reagan's campaign" where he observed Reagan's "common-sense approach to the problems of national taxes and spending."

"For eight years, I worked side by side with Gov. Reagan," said Deukmejian in his opening speech. He was Senate Republican floor leader at the end of his legislative career. "Together we worked on tax reform, welfare reform and we fought hard to get the death penalty put into law."

Both Curb and Deukmejian urge strong anti-crime measures, including appointment of tougher judges, the building of more prison cells, and a lessening of restrictions on evidence used in court. Both demand a loosening of environmental rules that they believe hinder the housing and road-building industries.

The agreement on issues might have been complete, except that Curb changed his mind about the Peripheral Canal, a proposed part of a state water project that would bring more fresh water from northern to southern California. Polls show a statewide majority supporting the project, particularly in the populous south, but Curb said he had concluded the costs of bonds to build the project would make the water too expensive. He urged unspecified "alternatives" while Deukmejian continued to favor the canal.

Curb's lead over Deukmejian in Mervin Field's California Poll has dropped from 16 percent to 2 percent since January. Curb has tried to hire White House western states political director Ed Rollins to repair his campaign. Both men still trail Bradley, Curb by 57 to 31 percent and Deukmejian by 55 to 31 percent, but a general election is expected to be much closer, with the Democratic Bradley perhaps vulnerable on the issue of crime.

Bradley's energetic campaigning, his trouble-free reign as mayor and his own devotion to budget cutting present the Republicans with a problem, so they try to damn with faint praise. Said Deukmejian: "He has been a good, ceremonial type mayor." CAPTION: Picture, LT. GOV. MIKE CURB. . . record $3.5 million for GOP primary