In May, only 9% of the state's likely Democratic voters in the June 2016 presidential primary said they would cast a ballot for Sanders. Since then, support for the Senator from Vermont has surged to 35% in the most recent poll.
While Clinton still leads Sanders, the Democratic front-runner and former secretary of state saw her popularity fall to 47% – a 19-point drop since May, according to the poll.
"Bernie Sanders has been making significant inroads across the country, and it's reflected here in California," Mark DiCamillo, the Field Poll's director, told the San Francisco Chronicle.
The new poll comes as Sanders reportedly will get his first congressional endorsement, with Arizona Representative Raul Grijalva expected to announce his backing at a Tucson rally on Friday. By comparison, Clinton has already garnered the endorsement of more than 100 members of Congress.
"Latino sectors of the country, African-Americans, people of color – they're feeling the economic pain as much as anyone else," Grijalva told the New York Times.
"Bernie has good positions on immigration and education, which are fountain issues for the Latino community. There's an opportunity to talk about those and expose the fact that Bernie is not just a one-tune candidate, which he's not."
The scandal surrounding Clinton's use of a private email account for official business during her time as secretary of state as threatened to engulf her campaign.
While the poll found less enthusiasm for Clinton as the party's nominee, it also found that 63% of likely voters support Vice President Joe Biden entering the race. However, only 15% of voters would back him.
"What that says to me is that Democratic voters really would like the opportunity to see their candidate against any and all comers, and Biden would certainly be welcomed into race," DiCamillo said. "That might turn more attention to the Democratic primary."