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.