Saying it was a complete “embarrassment” for the Democratic Party to lose working-class voters to Republican Donald Trump, Sen. Bernard Sanders on Thursday called on the party to remake itself and said he’ll back Rep. Keith Ellison as the next Democratic National Committee leader.

In an interview with Minnesota Public Radio, the Vermont senator and 2016 presidential hopeful made clear that he believes the party, under new leadership, must change dramatically.

“It is an embarrassment, I think, to the entire of Democratic Party that millions of white working-class people decided to vote for Mr. Trump, which suggests that the Democratic message of standing up for working people no longer holds much sway among workers in this country,” Mr. Sanders said.

The senator said he’ll support Mr. Ellison, who endorsed Mr. Sanders in the Democratic Party primary, to lead the DNC. Mr. Sanders had protracted fights with the DNC throughout the primary, accusing them of favoring Hillary Clinton — a claim that ended up proving true after hacked emails showed top DNC officials mocking the Sanders campaign and working on behalf of Mrs. Clinton.

The blatant favoritism led to the ouster of former DNC Chairwoman Debbie Wasserman Schultz on the eve of the Democratic Party convention in July. Ms. Wasserman Schultz was replaced by Donna Brazile, a former CNN contributor who came under fire after learned emails showed her feeding questions for a CNN town hall to Mrs. Clinton before the event.

The DNC leadership crisis, combined with the fact that the party was unable to get its base out to the polls on Election Day, has Mr. Sanders and other progressives calling for wholesale change.

“You cannot be a party which on one hand says we’re in favor of working people, we’re in favor of the needs of young people, but we don’t quite have the courage to take on Wall Street and the billionaire class,” Mr. Sanders said. “People do not believe that. You’ve got to decide which side you’re on.”