WASHINGTON — Unable once again to score a knockout, Senator 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.

Senator Clinton’s victory yesterday in Pennsylvania has only accentuated the quandary that Mr. Obama faces: Stay negative and he risks undermining the premise of his candidacy. Stay aloof and he underscores Mrs. Clinton’s argument that he will not be able to beat a “Republican attack machine” sure to greet him this summer.

The Obama campaign manager, David Plouffe, indicated last night which of those options 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 bruising 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 campaign. “It’s a real danger for Obama, and if you look at these recent ads, the messages they’re delivering in all these conference calls, it’s a far cry from last fall,” when the theme of hope emerged amid calls for a more negative tone, said Democratic consultant Steve Elmendorf, a Clinton supporter.

Republican strategist John Feehery put it less charitably: “That’s the danger of running as holier than thou. You have a lot farther to fall.”