Sen. Dianne Feinstein speaks Saturday at the 2018 California Democrats State Convention in San Diego. (Denis Poroy/AP)

California Democrats rebuked Sen. Dianne Feinstein at their annual convention this weekend, denying her the party’s endorsement in this year’s Senate race and giving a majority of their votes to her liberal primary challenger.

Just 37 percent of delegates to the statewide convention, held this year in San Diego, backed Feinstein in her bid for a fifth full term. More than 54 percent backed state Senate leader Kevin de León, who entered the race in October and has run to Feinstein’s left on health care, taxes and immigration. Candidates needed 60 percent of the vote to win the party’s endorsement, making Feinstein the first incumbent senator in recent memory who will run in June’s primary without official backing.

“California Democrats are hungry for new leadership that will fight for California values from the front lines, not equivocate on the sidelines,” de León said Sunday morning in a statement. “We all deserve a leader who will take our climate action to Washington and will fight each and every day to protect our human and civil rights, our immigrant families and Dreamers, champion universal healthcare and create good paying middle class jobs.”

Losing at the party’s convention does not stop any candidate from fighting to win in the primary. In 1990, as a candidate for governor of California, Feinstein was denied the party’s endorsement at the convention, in part due to her support for the death penalty. She went on to win the nomination, losing in November to Republican Pete Wilson.

Until Sunday morning, de León had little evidence that his challenge to Feinstein could succeed. The senator entered the year with more than $9.8 million in campaign funds; de León had just $359,261. A February poll from the Public Policy Institute of California found her leading de León by 29 points, albeit with 37 percent of voters undecided. And Feinstein, who since the start of her political career in San Francisco had crossed swords with her party’s left, had voted with the left of her Democratic caucus on issues involving the status of undocumented immigrants brought to the United States as children.

Sen. Dianne Feinstein (D-Calif.) spoke out against the Trump administration's recent steps to constrain immigration to the U.S. during a Senate Judiciary Committee hearing on Jan. 16. (Senate Judiciary Committee)

De León’s strong showing at the convention changed the narrative, demonstrating the trouble that Feinstein — who turns 85 this summer — will face in persuading a changing party to get behind her. The state senator has won the backing of more left-leaning unions, such as the Service Employees International Union and the California Nurses Association, and attacked Feinstein for conservative votes she cast after arriving in the Senate in 1993. (She is one of just five Democrats still in the Senate who voted for the Iraq War.)

The contest between Feinstein and de León has emerged as the only serious Democratic primary squabble, as Democrats in redder states such as Indiana and West Virginia are facing only token challenges. But even a bitter showdown in June may end up helping California Democrats. The party’s top-two primary system sends the two candidates who win the most votes in the summer to the general election in November — regardless of their party affiliation.

In 2016, that system locked Republicans out of a Senate race that was eventually won by Sen. Kamala D. Harris (D-Ca.). With Donald Trump leading the GOP ticket and no marquee race down the ballot, Democrats swamped Republicans that year, retaining a supermajority in the state legislature and carrying Orange County for the first time since the 1930s — helping Hillary Clinton carry several House districts once comfortably won by Republicans and nudging two of them into retirement.

No Republican running for Senate or governor in California this year has cracked single digits in public polls, raising the possibility of Democrat-only runoffs in November. That has left Republicans hoping that Democrats punch themselves out in crowded races for the Orange County seats. In the 39th District, being vacated this year by Rep. Edward R. Royce (R), eight Democrats and six Republicans are seeking runoff positions; in the 49th District, which Rep. Darrell Issa (R) nearly lost in 2016 before retiring this year, five Democrats are running against four Republicans.

The convention provided only a little clarity about the front-runners in those races. In the 49th District, Mike Levin — a first-time candidate backed by Democratic Party star Rep. Adam B. Schiff — won 53 percent of delegate votes, pushing past retired Marine Col. Doug Applegate, who nearly beat Issa but has struggled to raise funds this year. None of their rivals quit the race.