Senator Bernie Sanders’s campaign assailed the head of the Democratic National Committee on Wednesday, accusing her of blatantly favoring his rival, Hillary Clinton, during the party’s increasingly tense nominating battle.

The criticism comes amid growing fears about the prospects for party unity, as supporters of Mr. Sanders have been enraged by how they were treated at a state convention in Nevada last weekend and directed violent threats at the Democratic chairwoman there.

National party leaders, such as the D.N.C.’s chairwoman, Debbie Wasserman Schultz, have criticized how Mr. Sanders has handled the unrest, adding to the frustration within his campaign.

“The chairwoman of the Democratic National Committee, it is clear almost from the get-go that she has been working against Bernie Sanders — there’s no doubt about it, for personal reasons,” Jeff Weaver, Mr. Sanders’s campaign manager, said of Ms. Wasserman Schultz on MSNBC.