WASHINGTON  — Aides and supporters of Barack Obama and Hillary Clinton are searching for an alternative to Rep. Keith Ellison, the early favorite to become the next leader of the Democratic National Committee, amid resistance to the Minnesota liberal’s bid from key parts of the party’s base.

The contest to replace Florida Rep. Debbie Wasserman Schultz is evolving into a larger fight over the future of the party. Backers of Vermont Sen. Bernie Sanders are throwing their support behind Ellison while some Clinton supporters are searching for an alternative.

Ellison picked up an powerful endorsement Thursday from the AFL-CIO, which issued a statement calling him a “proven leader.”

But his candidacy remains under siege. Ellison has faced vocal criticism from prominent Democrats, Jewish groups and some union leaders, who have questioned his comments about Israel, his defense of Nation of Islam leader Louis Farrakhan and his commitment to his own party.

Last week, a union leader criticized the AFL-CIO for only including Ellison’s name, along with the choices to abstain or “make no endorsement at this time,” on the ballot sent to union members.

A federation faction “seems to want to push our movement further and further to the left,” Harold Schaitberger, president of the International Association of Fire Fighters, said in a statement Thursday. “That is a recipe for disaster as the most recent election results just showed.”

An editorial Wednesday in an official Nation of Islam publication, “The Final Call,” quoted articles that Ellison wrote in the 1990s praising Farrakhan as a “sincere, tireless and uncompromising advocate.”

The editorial accused Ellison, the first Muslim-American elected to Congress, of being a “hypocrite” for now making a “cowardly and baseless repudiation” of Farrakhan.