San Francisco — Gary Hart holds a slight 42 to 37 percent lead over Walter F. Mondale among California Democrats, according to The California Poll released today. Jesse Jackson trailed with 15 percent.
If the mood holds steady it means that the June 5 primary here could be the deciding factor in the presidential nomination.
Senator Hart's lead sharply contrasts with his distant sixth-place position in the last California Poll of candidate preferences Feb. 15, when Mr. Mondale had 46 percent and Hart only 5 percent. But it is unclear whether today's poll results indicate Hart is gaining ground among California Democrats or if his popularity is on a downturn after a high during early primaries, says Mervin D. Field, director of the poll.
The results of the poll are ''significant,'' however, Mr. Field says, because ''for Hart, winning this state would not only give his delegate total a big boost, but it would lend credence to a claim that after months of heavy campaign scrutiny, he was preferred in the nation's largest state, considered to be as representative of the country as any one state.''
Although Mondale leads today with slightly better than half the delegates required for nomination, campaign officials admit that California, with 10 percent of the delegates and a broad cross-section of constituencies, remains up for grabs.
''California is usually an advisory vote, a bandwagon vote. The nomination is usually sewn up'' by the time the June primary rolls around, Mondale campaign official Jonathan Kirsch says of the primary here. ''It's mathematically possible for Mondale to capture the nomination before California, but it's not politically likely, even with a strong showing in other states. Mondale will be close to a first-ballot nomination, and California can put him over the top. The circumstances make California crucial.''
Bert Coffey, a former state Democratic chairman running in the California primary as a Mondale delegate, says he's seen some Hart defections to the Mondale camp.
He offered the example of one strong Hart supporter in San Francisco who this week hinted that Hart might be a good vice-presidential candidate. Further, Mr. Coffey's view is that Mondale's special-interest-group support - labor and women , for example - will prove stronger than Hart's popularity among the ''yuppies, '' young urban professionals, who in today's poll account for a strong part of Hart's support.
Mr. Jackson's 15 percent showing represents a three-percentage-point increase in the last two months.
The poll was taken April 11 to 16 among 472 Democrats who said they were certain to vote in June. It has a margin of error of plus or minus 6.9 percent.
California's six million Democratic voters represent the diversity within the Democratic Party. They include strong union and minority ties, large numbers of young urban professional and elderly voters, as well as special-interest groups such as environmentalists and nuclear-freeze activists.
Predictably, the poll shows Hart preferred among those special-interest groups with whom he has faired well nationally.
Hart leads here in most demographic groups except minorities, where Jackson polled 42 percent and Mondale 36 percent, while Hart trailed with 20 percent. Minorities represent about 25 percent of the state's Democrats likely to vote.
Voters under age 50 strongly favor Hart, but those over 50 lean toward Mondale.
Hart's statewide lead over Mondale, the poll says, is a result of a 12-point lead in northern California. In southern California the two are virtually tied, with 40 percent for Hart and 39 percent for Mondale. Hart has a 10-point lead over Mondale among men, and just a two-point lead among women.
Mondale has a 12-point lead among voters with incomes less than $20,000, while Hart has a wide 27-point margin of support among those who make $40,000 or more.
''Many political observers feel that if Hart is able to win a number of late primaries, including California, a deadlock might occur at the convention whereby none of the three active candidates is able to muster a majority of delegates,'' says Mr. Field.
Among the possible compromise nominees, Sen. Edward Kennedy (D) of Massachusetts received an overwhelming 70 percent preference among the Californians polled. With Senator Kennedy removed from the list, New York Gov. Mario Cuomo led with 16 percent, followed by Texas Sen. Lloyd Bentsen with 9 percent.