WASHINGTON -- Unable to score a knockout, Sen. Barack Obama is likely to make his new negative tone even more negative -- with a sharp eye on trying to end the Democratic presidential nomination fight after the May 6 primaries in Indiana and North Carolina.

Sen. Hillary Rodham Clinton's victory yesterday in Pennsylvania has accentuated the quandary Mr. Obama faces: Stay negative and risk undermining the premise of his candidacy. Stay aloof and underscore Mrs. Clinton's argument that he will not be able to beat a "Republican attack machine" sure to greet him this summer.

Obama campaign manager David Plouffe indicated last night which option they would take. "We've done a lot of counterpunching. We've been swift and effective," he said. "For Democrats judging how we're going to perform as the nominee, we have been relentless."

But the candidate who rocketed to stardom as the embodiment of a new kind of politics -- hopeful, positive and inspiring -- saw his image tarnished in the fight for Pennsylvania. Provoked by Mrs. Clinton's repeated references to his remarks about the state's voters and her charges that he is an "elitist," Mr. Obama struck back in the closing days of the race.