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 votes to her liberal primary challenger, state Senate leader Kevin de León.

Feinstein will now face de León in a June primary that could define what the Democratic Party stands for in the age of President 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 León, 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 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.”

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 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 on hand; de León had just $359,261. A February poll from the Public Policy Institute of California found her leading de León 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.