His successful fund-raising has been led by Mary Pat Bonner, whose firm has been paid millions of dollars by Mr. Brock’s groups to court donors — some of whom have criticized the arrangement as well as Mr. Brock.

“He is a cancer,” said Mr. Morgan, who is close to Mr. Messina.

“If you care about your party and our country, you just do what you are asked,” said Mr. Morgan, referring to Mr. Brock’s public resignation from Priorities USA, which immediately reignited tales of infighting from Mrs. Clinton’s 2008 campaign. “If you care about yourself, you take your toys and go home.”

Mr. Brock declined to comment.

Susie Tompkins Buell, a friend of Mrs. Clinton’s and a donor from San Francisco who is close to Mr. Brock, said he “is an incredibly important part of the Democratic Party” whose work “protects us from the onslaught and destruction of the Republican attack machine.”

Ms. Buell added: “Certain people are trying to destroy David through off-the-record conversations with reporters. They are spineless and devious.”