Mr. McCain’s more focused assault comes after one of his worst weeks of the general election campaign, when he seemed to fumble for a consistent, overarching critique of Mr. Obama, who winged around the Middle East and Europe. Mr. McCain’s advisers continue to look for ways to bring more discipline to his message, and are being urged by some supporters to cut back the frequency of his question-and-answer sessions with reporters, a staple of his campaign but one that occasionally yields unscripted moments, misstatements and off-the-cuff pronouncements that divert attention from the themes he is trying to promote.

Image Senator John McCain, speaking Wednesday at a town-hall-style meeting in Aurora, Colo., has begun a campaign to define Senator Barack Obama in negative terms. Credit... Mary Altaffer/Associated Press

The intensity of the recent drive  which has included some assertions from the McCain campaign that have been widely dismissed as misleading  has surprised even some allies of Mr. McCain, who has frequently spoken about the need for civility in politics. The sentiment seeped onto television on Wednesday with Andrea Tantaros, a Republican strategist, saying on MSNBC that the use of Ms. Hilton in Mr. McCain’s commercial was “absurd and juvenile,” and that he should spend more time promoting his own agenda.

Mr. Obama’s campaign seized on those concerns, trying to turn the tables by portraying Mr. McCain as cranky and negative. The Democratic National Committee called Mr. McCain “McNasty.” Late Wednesday Mr. Obama released a counter advertisement citing editorials critical of Mr. McCain’s latest volley of attacks and featuring an announcer who says, “John McCain, Same old politics, same failed policies.”

Asked by reporters about Mr. McCain’s new advertisement, Mr. Obama said, “I do notice that he doesn’t seem to have anything to say very positive about himself.”

Mr. Obama’s chief strategist, David Axelrod, said that Mr. McCain’s strategy to define Mr. Obama negatively in voters’ minds, while similar to one that successfully worked against Mr. Kerry, would not work this year.

“When people are struggling, when they’re trying to pay their bills, when they’re concerned about their fundamental security, I don’t think they have much tolerance for Britney Spears and Paris Hilton,” Mr. Axelrod said. “I think they understand times are more serious than that, and they thought John McCain was, too.”

Mr. Schmidt, whom Mr. McCain placed in charge of day-to-day operations this month, specialized during the 2004 campaign in seizing on opportunities  think windsurfing; seemingly contradictory votes on Iraq policy  to paint Mr. Kerry negatively.