SAN DIEGO — Senator Dianne Feinstein suffered a setback in her effort to win a sixth term representing California as the state Democratic Party declined this weekend to endorse her re-election bid.

Ms. Feinstein is way ahead in most polls, and has a huge fund-raising advantage over her main opponent, Kevin de León, the California State Senate’s Democratic leader. Still, the vote here, at a raucous and well-attended party convention, is the latest indication of disenchantment with Ms. Feinstein, 84, among the party’s grass-roots advocates.

A candidate must garner the support of 60 percent of the delegates to win the party’s nomination. None of the candidates running for statewide election met that threshold.

Still, Ms. Feinstein’s showing was particularly stark given her status as a Democratic institution. Mr. de León drew 54 percent of the vote, or 1,508 votes, compared with 37 percent, or 1,023 votes, for Ms. Feinstein.