Feinstein will now face de Leon in a June primary that could define what the Democratic Party stands for in the age of President Donald Trump. The challenger is running on universal Medicare, free college tuition and other issues that have captivated the party's base. While Democrats in more conservative states have avoided primary challenges, activists see the California race as one of several where they can purify the party without risking a Republican win in November.
"It shows that the progressive arm of the Democratic Party is flexing its muscles," said Nina Turner, the president of Our Revolution, a group founded by Sen. Bernie Sanders, I-Vt., to elect left-wing candidates. "It's sending a message to the establishment that nobody is riding for free."
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 de Leon, who entered the race in October and has run to Feinstein's left on health care, taxes and immigration.
"It shows where the grass roots of the party is," said Rep. Ro Khanna, D-Calif., a congressional freshman who had urged Democrats to challenge Feinstein from the left. "She had a chance to make her case, Kevin had a chance to make his case, and the delegates spoke overwhelmingly for change. Kevin's been coming to this convention for the last 10 or 15 years, and this was the first time I've seen Sen. Feinstein show up."
Candidates needed 60 percent of the vote to win the party's endorsement, making Feinstein the first incumbent senator in decades who will run in the primary without official party backing. The size of the upset surprised some Democrats, as convention rules favor elected officials and their chosen delegates, and most of the party establishment had backed Feinstein. But starting last year, supporters of Sanders had organized to take over local parties, growing their strength at the state convention.
"California Democrats are hungry for new leadership that will fight for California values from the front lines, not equivocate on the sidelines," de Leon 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."
California candidates do not need their party's endorsement to run in and win primaries. In 1990, as a candidate for governor of California, Feinstein was denied the party's endorsement at the convention, thanks in part to her support for the death penalty. She went on to win the nomination, losing in November to Republican Pete Wilson.
But until Sunday morning, de Leon had little evidence that his challenge to Feinstein could succeed. The senator entered the year with more than $9.8 million in campaign funds on hand; de Leon had just $359,261. A February poll from the Public Policy Institute of California found her leading de Leon by 29 points, with 33 percent of likely voters undecided.
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. At the convention, Feinstein focused on gun safety, the issue that made her a national figure in 1978 after the assassination of George Moscone made her mayor of San Francisco.
"I authored the assault weapons ban that was law for 10 years," Feinstein told delegates. "Passing it now is my quest. It is my mission. I am absolutely dedicated to achieving this."
De Leon'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 has 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, and the only one who backed George W. Bush's 2001 tax cut. De Leon, who backed an effort to create "single-payer" universal health care in California, has also knocked Feinstein for opposing a Medicare-for-All bill introduced last year by Sanders.
"Kevin carried our single-payer bill, and that has become a litmus test across the country, whether people like it or not," said RoseAnn DeMoro, the executive director of the state nurses association. "Dianne Feinstein represents the most progressive city in America, and she doesn't support single-payer - that says everything. The establishment politics of Washington, D.C., are embodied by Dianne Feinstein."
Democrats, who took advantage of several Republican incumbent-versus-insurgent races over the past decade, now face several family squabbles of their own. In the Chicago suburbs, Rep. Daniel Lipinski, D-Ill., is being challenged over his opposition to abortion rights. In Maryland, Sen. Benjamin L. Cardin, D-Md., is being challenged by Chelsea Manning, a former Army private who was imprisoned after sending confidential government documents to WikiLeaks.
But the Democratic civil wars have been isolated to areas where any party nominee is favored to win, and where Republicans have not recruited strong challengers. Democrats in redder states, such as Indiana and West Virginia, are facing only token challenges; Rep. Collin C. Peterson, D-Minn., an antiabortion moderate seen as the only Democrat who can hold his rural district, has no primary challenger.
Even a bitter showdown between Feinstein and de Leon in June may end up helping California Democrats, thanks to state election laws. The party's top-two primary system sends the candidates who win the most votes in the summer to the November general election regardless of their party affiliation. No serious Republican challenger has emerged to take advantage of the Democratic split; Gary Coson, the one Republican contender who has filed a campaign finance report, had just $130 on hand at the start of the year.
In 2016, that system locked Republicans out of a Senate race that was eventually won by Sen. Kamala D. Harris, D-Calif. With 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. This helped Hillary Clinton carry several House districts once comfortably won by Republicans and nudged two of them into retirement.
No Republican running for Senate or governor in California this year has cracked double digits in public polls, raising the possibility of Democrat-only runoffs in November. And Turner scoffed at the idea that a primary in California could hurt the party.
"It's called democracy," said Turner. "Last time I checked, in America, we run for office. It may not feel good if you're an incumbent, but you better get used to it."