Senator Dianne Feinstein is widely expected to preserve her seat in the midterm elections, but the support for Mr. de León reflects the rise of younger liberal activists.
LOS ANGELES — Six weeks after winning an overwhelming victory among California primary voters, Senator Dianne Feinstein suffered an embarrassing setback Saturday night as the state Democratic Party officially endorsed her rival for election this November.
Kevin de León, the Democratic leader in California’s State Senate, won the support of the party leadership despite an effort by Ms. Feinstein to convince the party not to offer an endorsement.
The vote for Mr. de León reflected the rise of younger liberal activists in the California Democratic leadership, some of whom regard Ms. Feinstein as a moderate compared with the intensely progressive voices who are coming up through the party ranks. These activists tend to be younger and more left-leaning than the state party at large.
Mr. de León, 51, remains a long-shot candidate in his two-way race against the 85-year-old Ms. Feinstein in November; the two Democrats are facing each other, with no Republican in the mix, after they prevailed in the state’s so-called “top two” open primary last month. Still, the vote for Mr. de Leon was a reminder of Ms. Feinstein’s tepid support among liberal Democrats as the party debates how to take on President Trump in the fall midterm elections.
In a tweet after the endorsement, Mr. de León wrote: “Earning the endorsement of so many leaders and activists of the @CA_Dem 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.”
Mr. de León’s victory was the latest of several triumphs for liberals in Democratic primary elections this year. Most notably, Alexandria Ocasio-Cortez, a New York community organizer, scored a stunning upset over Representative Joseph Crowley, a Democratic Party leader and 19-year incumbent, in his bid for renomination last month. Ms. Ocasio-Cortez is the most prominent of several members of the Democratic Socialists of America who have won primaries for state and local offices this year.
Two other progressives, Jared Polis of Colorado and Ben Jealous of Maryland, also beat challengers last month for the Democratic nomination for governor in those states.
Ms. Feinstein handily defeated Mr. de León in the California Senate primary last month, receiving 44.2 percent of the vote to his 11.5 percent. She carried every county of the state.
