Field Poll finds Clinton’s California lead down to tiny margin

Democratic presidential candidate Hillary Clinton campaigns at the Golden Dome Athletic Center at Rutgers University Student Health in Newark, N.J., on Wednesday, June 1, 2016. (Carolyn Cole/Los Angeles Times/TNS) less Democratic presidential candidate Hillary Clinton campaigns at the Golden Dome Athletic Center at Rutgers University Student Health in Newark, N.J., on Wednesday, June 1, 2016. (Carolyn Cole/Los Angeles ... more Photo: Carolyn Cole, TNS Photo: Carolyn Cole, TNS Image 1 of / 5 Caption Close Field Poll finds Clinton’s California lead down to tiny margin 1 / 5 Back to Gallery

Hillary Clinton’s lead has withered to the narrowest of margins over Sen. Bernie Sanders in what is shaping up to be a ballot-box barn burner in Tuesday’s California Democratic primary, a new Field Poll showed Thursday.

Forty-five percent of likely primary voters surveyed said they are siding with Clinton, compared with 43 percent for Sanders, according to the poll. That’s down from the former secretary of state’s 11-point lead in the January Field Poll and six-point lead in April. Her lead over Sanders was 73 to 10 percent when the Field organization conducted its first poll for this presidential campaign cycle in February 2015.

Mark DiCamillo, director of the Field Poll, noted that the latest results put Clinton’s lead within the margin of error. “The election is up for grabs,” he said.

It’s the second poll in the past week to show the race in a virtual dead heat. A Public Policy Institute of California survey last week also showed Clinton clinging to a two-point advantage.

Clinton is certain to have more delegates than Sanders after the primary, even if she loses. But a California victory would help Sanders’ argument that he’s more electable than Clinton in a November matchup with presumed Republican nominee Donald Trump.

The new poll will do nothing to discourage Sanders in that regard. In theoretical November matchups that sampled California voters of all stripes, he leads Trump by 29 points, compared with 19 points for Clinton.

The poll also reinforced splits that have been evident throughout the campaign. It shows Sanders winning voters under age 30 by a 5-to-1 ratio, while those over 65 favor Clinton by 2 to 1.

Clinton has leads of 49 to 40 percent among both women and registered Democrats, while men back Sanders by 48 to 39 percent. Decline-to-state voters, who can vote in the Democratic primary, back Sanders by 54 to 27 percent, according to the poll.

“Clinton does lead among the registered Democrats, but Sanders is getting very close because of the strong support of the independents,” DiCamillo said.

The poll also showed that even as the candidates travel around the state, many people have already cast a ballot. Twenty-three percent of those surveyed had already voted early or by mail. Among those voters, Clinton leads Sanders by nine points, DiCamillo said.

Among those who had not yet voted, 45 percent back Sanders and 44 percent are behind Clinton.

There’s one gap that isn’t narrow: the difference in enthusiasm levels. Only 45 percent of likely Clinton voters characterized themselves as “enthusiastic” supporters, compared with 65 percent of Sanders supporters.

As for Republicans, the poll indicates they are overcoming earlier qualms about Trump. Sixty percent of those surveyed said they had a favorable impression of him, up seven points since April.

DiCamillo said the California primary is a crucial gauge in the presidential race.

“If Clinton were to win in California, that might give her enough impetus to say it’s time for Sanders to throw his support behind her,” he said. “If Sanders were to win the state, that would have symbolic value and allow him to argue that this should be decided at the convention.

“It will boil down to turnout,” he said. “If Sanders can get his voters to the polls, he could pull out a victory in California.”

The survey of 1,002 voters was conducted May 26-31 and has a margin of error of 3.2 percentage points.

Peter Fimrite is a San Francisco Chronicle staff writer. Email: pfimrite@sfchronicle.com. Twitter: pfimrite