Top California Democratic Party officials jolted four-term incumbent Sen. Dianne Feinstein with a vote to endorse her November election opponent, fellow Democrat state Sen. Kevin de León.

De León won 65 percent of the 333 ballots cast by members of the party’s executive board at an Oakland hotel Saturday, while 28 percent preferred “no endorsement,” which Feinstein requested last week. Feinstein won only 7 percent of the vote. The threshold for winning an endorsement was 60 percent.

VOTING RESULTS — U.S. Senate:



Feinstein — 22 (7%)

de León — 217 (65%)

No Endorsement — 94 (28%) pic.twitter.com/BoxkNIk4sM — California Democratic Party (@CA_Dem) July 15, 2018

“Earning the endorsement of so many leaders and activists of the California Democratic Party isn’t just an honor and a privilege; today’s vote is a clear-eyed rejection of politics as usual in Washington, D.C.,” de León said.

Feinstein’s campaign manager, Jeff Millman, downplayed the result.

“While 217 delegates expressed their view today, Sen. Feinstein won by 2.1 million votes and earned 70 percent of the Democratic vote in the California primary election, carrying every county by double digits over her opponent,” Millman said. “We are confident that a large majority of California Democrats will vote to re-elect Sen. Feinstein in November.”

The party’s surprising rebuke to one of the nation’s best-known politicians breathes life into de León’s flagging campaign.

Feinstein still has a commanding lead in the polls and far more money to spend. But winning the state party’s endorsement means de León will have his name and photo on door hangers, slate mailers and email blasts sent to voters along with other candidates the party is promoting, including gubernatorial hopeful Gavin Newsom. That outreach “will top 2 million people for sure,” party chair Eric Bauman said.

The win could also lead to a huge money boon for de León’s campaign, which had $693,689 cash on hand as of May 16, the date of the most recent campaign finance report. Now his campaign will be able to jointly raise money with the party.

De León nearly landed the party’s endorsement at its February convention when he received the support of 54 percent of the delegates — just short of the 60 percent threshold. That was a vote of roughly 2,800 party activists, compared with the 333 executive board members who voted Saturday.

Party leaders acknowledged that endorsing de León after Feinstein easily won the primary could send mixed signals to voters.

“It is a concern that it will be confusing,” Bauman said.

The activists who vote at party functions are generally more left-leaning than the larger universe of Democratic voters. Unlike Feinstein, de León supports single-payer health care and opposes extending permission for the National Security Agency to obtain Americans’ email and other personal records without obtaining a warrant. Both stances endear him to the party’s left wing.

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Feinstein initially sought the party’s backing, then encouraged delegates to vote “no endorsement” in a minute-long video her campaign presented Saturday. Feinstein said she didn’t think winning the endorsement was crucial to her success in November.

“I don’t think it is, to be honest with you,” Feinstein said before the votes were counted. “This wasn’t a close primary.”

It wasn’t, as Feinstein received 70 percent of the votes cast for Democrats in the 32-candidate primary field in June and and carried every county in California. She won 44 percent of the overall vote, while de León squeaked into second place with 12 percent.

Plus, not only does Feinstein’s campaign have $7 million cash on hand, she is one of the wealthiest members of Congress with a minimum net worth of $58 million, according to a March analysis by Roll Call magazine.

Nevertheless, for the past several weeks, Feinstein tried to win the endorsement by telling delegates that her four terms in the Senate has enabled her to reach leadership positions.

Asked Saturday what it would mean if the party overlooked those accomplishments and endorsed de León, Feinstein said, “That thought has occurred to me, but I wiped it out of my mind quickly.”

On Saturday, she pointed out to attendees at the party’s Women’s Caucus meeting that she is the top Democrat on the Senate Judiciary Committee as the panel prepares for “one of the biggest moments this country has had” — the panel’s confirmation hearings for President Trump’s Supreme Court nominee, appeals Judge Brett Kavanaugh.

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For his part, de León tried to walk a line between honoring Feinstein’s service and suggesting that it was “time for a new generation” of leadership. Speaking after her at the Women’s Caucus, de Leon countered Feinstein’s seniority argument by urging Senate Democrats to take bolder action to derail Kavanaugh’s nomination.

Joe Garofoli is The San Francisco Chronicle’s senior political writer. Email: jgarofoli@sfchronicle.com Twitter: @joegarofoli