“Kevin de León represents the future of the Democratic Party — a solidly progressive legislator who can see that things like Medicare for all and moving forward on climate justice are the things that really energize our base in the long run,” said Daraka Larimore-Hall, 44, a Santa Barbara community college professor and a member of the executive committee who voted for Mr. de León.

The vote means that Mr. de León’s name will appear on millions of pamphlets that the Democratic Party will mail to its voters this fall. He will also appear in the state’s official voting guide as the Democratic Party candidate for Senate, which could make a difference for so-called low-information voters not following the race as they seek to decide whom to support.

However, in a year when Democrats are battling to capture as many as seven Republican-held congressional seats in California — a key part of the party’s national strategy to win the House — officials said it was highly unlikely the party would spend any money on Mr. de León’s behalf, since both candidates in the November contest are Democrats. No Republican candidate is on the Senate ballot because Ms. Feinstein and Mr. de León prevailed in the state’s “top two” open primary last month.

And Ms. Feinstein goes into the general election with huge advantages over Mr. de León.

She is a California institution, who has consistently raised more funds than Mr. de León, and by big margins. She has the support of some of the most influential national Democrats in the nation: former President Barack Obama, Gov. Jerry Brown and Senator Kamala Harris among them.

By many indications, the executive committee of the California Democratic Party is also not reflective of the electorate at large. Mr. de León, the former leader of the State Senate, lost to Ms. Feinstein in every county in the state in the June primary; he even lost his home Senate district in Los Angeles. A U.S.C. Dornsife/Los Angeles Times poll in June showed Ms. Feinstein leading Mr. de León by 36 percent to 18 percent among registered voters, though nearly 50 percent said they were undecided.