Senators Hillary Rodham Clinton and Barack Obama veered from collegial to clenched and combative in a debate on Thursday, with Mrs. Clinton turning especially aggressive as she all but accused Mr. Obama of plagiarism and derided his political message as “change you can Xerox.”

Mr. Obama, buoyed by 11 straight victories in the most recent nominating contests, sought to maintain a positive tone throughout, though at one point he accused Mrs. Clinton of suggesting that his supporters were “delusional” or “being duped” by his themes of hope and unity.

Those few sharp rejoinders, as well as an emotional closing comment by Mrs. Clinton that seemed to move the audience  and even Mr. Obama  were the most memorable moments in a debate that had been loaded with expectation, with Mrs. Clinton seeking to stop the Obama juggernaut.

After the 90 minute face-off was over, it was not clear that Mrs. Clinton, in the toughest position of the campaign for her, had done enough to change the course of the contest.