Taking his outsider message into the heart of the Democratic establishment, Senator Bernie Sanders, the Vermont independent, challenged hundreds of the party’s leaders on Friday to embrace his candidacy, warning that the huge crowds of supporters he has drawn may not vote for Democratic candidates in 2016 unless he is at the top of the ticket.

Mr. Sanders, who has been gaining ground in the polls, was warmly complimentary at first in addressing the Democratic National Committee’s summer meeting here, crediting the party faithful for fighting on behalf of working people and low-income Americans.

But he soon turned pointed, suggesting that the Democrats’ electoral losses in 2014 – when Republicans won control of the Senate – could be repeated if the party nominated a traditional politician.