A new poll shows San Diego Republican John Cox holds a seven-point advantage for second place among likely voters in the governor's election Tuesday, signaling that voters prefer him and Democratic Lt. Gov. Gavin Newsom to face off in the November general election.

The poll by UC Berkeley's Institute of Governmental Studies also found Newsom expanding his lead, with 33 percent of likely voters backing the San Francisco Democrat. Support for Cox, a businessman, grew to 20 percent, while former Los Angeles Mayor Antonio Villaraigosa's backing grew to 13 percent. The percentage of undecided voters dropped from 13 percent in April to 7 percent in the current poll.

"We think it's likely to be Cox against Newsom in the general election," said Mark DiCamillo, director of the poll.

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The poll, conducted after President Donald Trump's endorsement of Cox, showed Republican support coalescing for Cox over the other leading Republican in the race, Assemblyman Travis Allen of Huntington Beach. In April, they were nearly neck-and-neck, with Cox at 47 percent and Allen at 38 percent among Republicans. The latest poll had Cox at 53 percent to Allen's 32 percent.

"It seems Trump's endorsement put Cox over the top," DiCamillo said. "He's the president, and the Republican Party is very much supportive of Donald Trump. It's his party these days, so when Trump weighs into the California gubernatorial primary, I think that has an impact on Republican voters."

After his initial endorsement, Trump tweeted again Monday saying "California has a rare opportunity to turn things around and solve its high crime, high tax problems...vote for GOP Gubernatorial Candidate JOHN COX, a really good and highly competent man. He'll Make California Great Again!"

Overall, 12 percent of likely voters preferred Allen, while state Treasurer John Chiang remained at 7 percent and former state schools chief Delaine Eastin held steady at 4 percent.

DiCamillo noted the poll is yet more good news for Newsom, who said this week on a campaign bus tour across California that Democrats should unite behind one gubernatorial candidate rather than fighting it out in an expensive and divisive Democrat-on-Democrat competition in the general.

"It did appear early on that there was some chance that Villaraigosa might make it," DiCamillo said. "That would certainly add a lot of drama to the general election, but in a traditional Democrat against a Republican race, in this state, the Democrat would be heavily favored."

Newsom led the field among Democrats, with 50 percent of likely voters saying they prefer him to the other major choices in the party. Villaraigosa was next, with 20 percent of Democratic support.

The poll was conducted from May 22 to 28, with 23 percent of likely voters sampled having already cast votes. Among the early voters, 38 percent said they cast votes for Newsom, while 23 percent had voted for Cox.