alifornia's largest grass-roots Republican organization has dealt a surprising blow to the U.S. Senate candidacy of Rep. Barry Goldwater Jr., whom polls show leading all contenders to fill the seat being vacated by Sen. S.I. Hayakawa.

Rep. Robert K. Dornan, an energetic former television talk show host, won the endorsement of the California Republican Assembly Sunday with 196 votes. Goldwater got only 31.

Although Goldwater aides immediately discounted the significance of the endorsement, the enormous vote of confidence from a group representing 14,000 conservative campaign workers gave Dornan a tremendous lift and cheered other Republicans running behind the son of conservative Republican patriarch Barry M. Goldwater Sr. of Arizona.

Dornan's media representative, Milan Radovich, said the vote showed Republicans are becoming disgruntled with Goldwater's reluctance to debate his opponents. "If you can find Barry, you'll find Jimmy Hoffa and Judge Crater and everybody else," Radovich said.

Statewide polls show Dornan running fifth in a field of eight for the Republican nomination and the right to meet the likely Democratic nominee, Gov. Edmund G. (Jerry) Brown Jr.

But Dornan has insisted that his mail-order fund-raising, frequent television appearances and lively campaign style would cut into Goldwater's support. Dornan volunteers telephoned delegates to the assembly's convention for three weeks before it convened Saturday in Fresno.

Meanwhile, the convention failed to endorse a candidate for governor when state Lt. Gov. Mike Curb fell two votes short of the two-thirds majority required for an endorsement. Curb, the front-runner among conservatives, won 198 votes against 102 for state Attorney General George Deukmejian.

A Mervin Field California Poll released in February showed Goldwater leading in the Senate primary race with 34 percent, followed by 18 percent each for San Diego Mayor Pete Wilson and Rep. Paul N. (Pete) McCloskey, 9 percent for presidential daughter and businesswoman Maureen Reagan and 6 percent for Dornan. A new poll is expected to be released Tuesday.

Ben D. Key, Goldwater's campaign coordinator expressed confidence that Goldwater, who also leads Brown in the polls, still would win the primary, and, "We look forward to Mr. Dornan's campaigning for Mr. Goldwater. . . ."

Republican historians noted that the assembly endorsement boosted the 1966 Reagan campaign for governor, and helped conservative Max Rafferty upset Sen. Thomas Kuchel in the 1968 primary. Hayakawa, however, won the 1976 primary after being virtually ignored by the grassroots group.