A new Field Poll shows California Attorney General Jerry Brown with a strong lead in next year’s race for governor, even before he’s declared himself a candidate and despite months of campaigning by his Democratic rival and three Republicans vying for their party’s nomination.

The poll released Thursday found that Brown, the former governor, or fellow Democrat Gavin Newsom, the mayor of San Francisco, would fare well against any of the three Republicans.

It found that half of Republican voters had yet to make up their minds in the contest between former eBay chief executive Meg Whitman, former congressman Tom Campbell and state Insurance Commissioner Steve Poizner.

Among Democrats, Brown has a comfortable 20-point lead over Newsom with a quarter of voters undecided. U.S. Sen. Dianne Feinstein would trounce either if she opted to get into the race, the poll found.

The poll underscores the struggles of Newsom’s campaign. He trailed Brown nearly 7-to-1 in fundraising during the first half of the year, even before Brown announced an exploratory committee last week that will allow him to collect larger campaign donations.

Newsom has been campaigning statewide for months as he tries to move beyond his image as the mayor who opened his city to gay marriage. Brown has repeatedly hinted at seeking the office he held from 1975 to 1983, but has so far remained coy.

Newsom is seeking to improve his standing in Southern California, where the poll shows Brown has a commanding 31-point lead. He tried to tap into the Hollywood donor base this week at a Los Angeles fundraiser with former President Bill Clinton.

“The biggest differences in this contest, however, are by age,” pollsters Mark DiCamillo and Mervin Field wrote. “Among younger voters in the 18-39 age group, Newsom leads Brown 41 percent to 32 percent. However, Brown leads Newsom by 30 points among voters age 40 to 64, and by a huge 45 points among seniors age 65 or older.”

Older voters are considered among the most reliable to turn out, especially in a primary election.

Thursday’s Field Poll was based on 1,005 interviews with registered voters conducted by phone from Sept. 18 to Oct. 5. It has a sampling error rate of plus or minus 3.2 percentage points for all voters, and a sampling error rate of plus or minus 4.5 percentage points for subsamples of Democrats and Republicans.

The poll found that 49 percent of GOP voters are not ready to favor a particular candidate, unsurprising with the election so far away. The primary is next June.

Among those who do have a preference, Whitman and Campbell were in a virtual tie, at 22 percent and 20 percent respectively. Poizner trailed with just 9 percent support among GOP voters surveyed.

The poll found Whitman with a slim lead among Southern California voters and Campbell with a slim lead in the north. Campbell is more popular with voters aged 18 to 49, while Whitman appeals to those ages 50 to 64. Older Republicans were evenly split, the poll found.

Any of the three would have a tough time against a Democratic opponent in the general election, the poll found.

Despite his lackluster fundraising numbers to date, even Newsom has leads of 5 to 9 percent against any of the GOP candidates in the traditionally Democratic state.

Still, Brown and Newsom don’t come closing to having Feinstein’s appeal, according to the poll: 40 percent of Democrats said they would back her if she were to jump in, compared with 27 percent for Brown and 16 percent for Newsom.

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Field Poll: www.field.com