After spending much of the summer searching for an effective line of attack against Senator Barack Obama, Senator John McCain is beginning a newly aggressive campaign to define Mr. Obama as arrogant, out of touch and unprepared for the presidency. 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.

On Wednesday alone, the McCain campaign released a new advertisement suggesting — and not in a good way — that Mr. Obama was a celebrity along the lines of Britney Spears and Paris Hilton. Republicans tried to portray Mr. Obama as a candidate who believed the race was all about him, relying on what Democrats said was a completely inaccurate quotation. The Republican National Committee began an anti-Obama Web site called “Audacity Watch,” a play on the title of Mr. Obama’s book “The Audacity of Hope.” And, in a concerted volley of television interviews, news releases and e-mail, campaign representatives attacked him on a wide range of issues, including tax policies and energy proposals. The moves are the McCain campaign’s most full-throttled effort to define Mr. Obama negatively, on its own terms, by creating a narrative intended to turn the public off to an opponent.

The run of attacks against Mr. Obama over the last couple of weeks have been strikingly reminiscent of that drive, including the Bush team’s tactics of seeking to make campaigns referendums on its opponents — not a choice between two candidates — and attacking the opponent’s perceived strengths head-on. Central to the latest McCain drive is an attempt to use against Mr. Obama the huge crowds and excitement he has drawn, including on his foreign trip last week, by promoting a view of him as more interested in attention and adulation than in solving the problems facing American families.

“I would say that it is beyond dispute that he has become the biggest celebrity in the world,” Mr. Schmidt said in a conference call with reporters on Wednesday. “The question that we are posing to the American people is this: ‘Is he ready to lead yet?’ And the answer to the question that we will offer to the American people is: ‘No he is not.’ ”

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.”

Seeking similar openings, the campaign seized on Mr. Obama’s decision to skip a visit with wounded United States troops in Germany. (The McCain campaign said Mr. Obama canceled because he could not take the news media with him to the hospital, an assertion denied by the Obama campaign and undercut by the accounts of reporters.) The new focus has been welcomed by some Republicans. “They’re now in a position of driving news as opposed to reacting to it,” said Brian Jones, a former aide to Mr. McCain.

But some fear a backlash. And Mr. McCain does not like to follow a script. People who know him said that it may be a challenge to apply the Bush model — strict adherence to the message of the day by the candidate combined with a relentless drive to define the opponent negatively — to a campaign not known so far for discipline or consistency.