Mr. Mitchell brushed off the possibility that the group’s endorsement would be seen as a sign of a splintering of the progressive left. The vote among “tens of thousands” of party members and national committee leaders resulted in a commanding majority for Ms. Warren, a party spokesman said; she received more than 60 percent of the votes on the first ballot.

But the announcement was met with derision from some of Mr. Sanders’s supporters. The national committee leaders — 56 people — held 50 percent of the voting power, with party members accounting for the other 50 percent. Several Sanders supporters called for the Working Families Party to release the full vote totals, which it has declined to do.

Mr. Sanders’s campaign shook up its New Hampshire staff over the weekend as Ms. Warren continued to make inroads among progressives. Ms. Warren is coming off a debate performance last week that was generally well received, and she held a rally in New York on Monday that drew thousands to Washington Square Park.

Also on Monday, Ms. Warren unveiled a plan to combat corruption in government, a core theme of her campaign. The plan is based on a wide-ranging anticorruption bill that she first proposed last year and is a cornerstone of her stump speech on the campaign trail.