The Reverend Al Sharpton, one of the most prominent black civil rights activists in the country, is challenging the policies of the only African-American candidate for president.

Rev. Sharpton told CBS News yesterday that he wanted Senator Obama, a Democrat of Illinois, to explain why he backed Senator Lieberman when the Connecticut lawmaker was locked in a tight race for the Democratic Party nomination for the Senate last year.

“Why shouldn’t the black community ask questions? Are we now being told, ‘You all just shut up?” CBS quoted Rev. Sharpton as saying. “Senator Obama and I agree that the war is wrong, but then I want to know why he went to Connecticut and helped Lieberman, the biggest supporter of the war.”

Rev. Sharpton’s decision to criticize Mr. Obama’s support last year for Mr. Lieberman is an interesting one. Mr. Lieberman, who was elected as an independent but is still caucusing with the Democrats, has a tenuous relationship with his former party. But he is crucial to the Democrats because he holds the key to the party’s Senate majority.

The sideswipe makes clear that Rev. Sharpton, who sought the Democratic presidential nomination unsuccessfully in 2004, will not give Mr. Obama a pass simply because he is black. Indeed, a story citing anonymous sources in the New York Post yesterday said Rev. Sharpton was jealous of Mr. Obama’s popularity among Democrats.

On CBS News, Rev. Sharpton denied that he was jealous of Mr. Obama. Rev. Sharpton also suggested that the Post story came from the senator’s campaign, which was seeking to pressure him into an endorsement.

Both Mr. Obama and Senator Clinton, the two leading candidates in the Democratic presidential field, are trying to lock up support from black leaders.

Earlier this month, both candidates were in Alabama and appeared at black churches. Mr. Obama’s presence in the race has threatened to siphon away support in the African-American community that many political analysts believe would otherwise have gone to Mrs. Clinton.