Though Mr. Lewis had praise for Mrs. Clinton and for her historic candidacy, he said he could decide within days whether to formally endorse Mr. Obama.

He also said he and other lawmakers would meet in the coming days to decide how they intended to weigh in on the nominating fight. If neither Mrs. Clinton nor Mr. Obama receive enough pledged delegates to win the nomination, superdelegates like Mr. Lewis may play the deciding role in who wins.

“If I can be used as a mediator, a negotiator or a peacemaker, I’d be happy to step in,” Mr. Lewis said, adding that he intends to speak to both candidates in hopes of ending the race amicably in the next month. “I don’t want to see Mrs. Clinton damaged or Mr. Obama damaged.”

Jay Carson, a spokesman for Mrs. Clinton, said Thursday: “Congressman Lewis is a true American hero, and we have the utmost respect for him and understand the great pressure he faced. And Senator Clinton enjoys incredibly strong support from superdelegates around the country from all regions and races.”

Image Hillary Rodham Clinton shows off autographed boxing gloves from the boxer Kelly Pavlik while speaking Thursday in Ohio. Credit... Béatrice de Géa for The New York Times

The comments by Mr. Lewis underscored a growing sentiment among some of the party’s black leaders that they should not stand in the way of Mr. Obama’s historic quest for the nomination and should not go against the will of their constituents. As superdelegates, they may have the final say, which is something Mr. Lewis said he feared would weaken Democrats and raise Republicans’ chances of winning the White House.

Still, the Democratic nominating fight clearly has many turns ahead. On Thursday, Mrs. Clinton unleashed the most ambitious mobilization of her forces in weeks, reflecting the intense pressure she is under from Mr. Obama, the political necessity for her of towering performances in the delegate-rich primaries in Ohio and Texas on March 4, and her fresh hope of an upset victory in Wisconsin.