Mr. Ellison said that the release of the audiotape — by the Investigative Project on Terrorism, a nonprofit research group in Washington — amounted to “an attempt by right-wing interests to drive a wedge between longstanding allies in the fight for equal rights.”

Mr. Ellison’s staff members sent reporters materials from the Southern Poverty Law Center describing the head of the project as an anti-Muslim extremist.

CNN’s website may have inflamed matters further on Thursday when it excerpted an article Mr. Ellison wrote while he was in law school at the University of Minnesota. Mr. Ellison, writing under the name Keith E. Hakim in the student newspaper, defended the right to voice diverse opinions on Zionism, saying that in opposing an anti-Zionist speaker, “the University’s position appears to be this: Political Zionism is off limits no matter what dubious circumstances Israel was founded under; no matter what the Zionists do to the Palestinians; and no matter what wicked regimes Israel allies itself with — like South Africa. This position is untenable.”

Jewish organizations are not the only ones opposing Mr. Ellison. Leading Democrats, including some of President Obama’s loyalists, want a full-time party leader and are hoping others will enter the race, which will be decided by a vote of party committee members in February.

Despite quickly winning support from Mr. Schumer and other liberal titans such as Senators Bernie Sanders of Vermont and Elizabeth Warren of Massachusetts, Mr. Ellison has drawn increasing skepticism.

Firefighters, some building trades unions and the food and commercial workers have all used recent meetings of the A.F.L.-C.I.O. to advocate against making what they see as an overly hasty endorsement of Mr. Ellison’s D.N.C. candidacy by the labor federation, according to several Democratic officials familiar with the discussions.

Mr. Ellison, however, enjoys significant support from a group of powerhouse union leaders, including Randi Weingarten, president of the American Federation of Teachers, and Lee Saunders, president of the American Federation of State, County and Municipal Employees. And Richard L. Trumka, president of the A.F.L.-C.I.O., signaled the federation’s inclination to support Mr. Ellison on Wednesday when he emailed union leaders a ballot that included just three options: Mr. Ellison, No Endorsement and Abstain.