The McCain campaign and Republican Party rolled out a concerted offensive to allege that Obama, fresh from an overseas tour, was keener to soak in the adulation of adoring crowds than to offer real solutions for US voters.
The Illinois senator hit back to accuse a rattled McCain of promoting no positive ideas about how he might address challenging issues such as the faltering US economy and the Iraq war.
"It's beyond dispute that he's become the biggest celebrity in the world," Steve Schmidt, a senior adviser to McCain, told reporters as the campaign unveiled a national television ad called "Celeb."
"The question we are posing to the American people is this, is he ready to lead yet? The answer that we will offer to the American people is, no he is not," he said.
The ad features images of the pop diva Spears and the socialite Hilton, both known as much for their off-field antics as for their professional activities, and a sea of 200,000 people listening to Obama speak in Berlin last week.
It juxtaposes those images with claims that the Democrat's policies would raise taxes and increase imports of foreign oil.
"I don't pay attention to John McCain's ads, although I do notice that he doesn't seem to have anything very positive to say about himself, does he?" Obama told reporters in Missouri.
"He seems only to be talking about me. You need to ask John McCain what he is for, not just what he is against," he said, as the Republican ran into flak for a no-holds-barred offensive of recent days.
In other ads, McCain has accused Obama of being responsible for higher fuel prices because of his opposition to offshore oil drilling. The Republican has also said his opponent would rather lose the Iraq war than the election.
Responding to the "Celeb" ad, the Obama campaign highlighted a report in Wednesday's New York Times that warned about the dangers of negative attacks this far out from the November election.
The newspaper quoted Todd Harris, a Republican strategist who worked for McCain during his 2000 presidential run, as saying this year's election was turning into a referendum on Obama's fitness to lead.
"So being aggressive against Obama on questions of leadership and trust and risk are important, but at the same time I think they need to be very careful because McCain is not at his best when he is being overly partisan and negative," Harris said.
According to a CNN/Opinion Research poll Wednesday, 40 per cent of respondents believe McCain is attacking Obama unfairly, nearly double the number who find Obama's attacks on McCain unfair.
But the poll also said 37 per cent found Obama arrogant, and 44 per cent felt the Democrat was acting as if he has already won.
The Washington Post meanwhile concluded that another McCain offensive of recent days, that Obama refused to visit wounded US troops in Germany as he was barred from bringing a media entourage, has "no evidence" to back it up.
Obama's chief strategist David Axelrod told MSNBC the McCain barrage was "baloney."
"I am going to paraphrase, oops he did it again," he said, playing off the title of a Britney Spears album. "This is not the John McCain we expected."
Undeterred by the media criticism of McCain's more strident tactics, the Republican National Committee launched a website called "Obama Audacity Watch."
The RNC said its website highlights instances of Obama prematurely donning a presidential mantle, including reports that Obama is already planning his transition to the Oval Office well before November's vote.
McCain's campaign manager Rick Davis insisted the Republican hopeful was merely setting the record straight after initial attacks from Obama.
"I'm going to let the American people decide what they think is negative or not negative, but I'm going to do everything in my power to protect my candidate," he said.