John McCain on Thursday defended his campaign's new television ad likening Barack Obama to Britney Spears and Paris Hilton and suggesting his Democratic rival is more about flash than substance.

"We are proud of that commercial," McCain told an Obama backer who described the ad as "below" the Arizona senator.

"We think Americans need to know that I believe that we should base this campaign on what we can do for Americans here at home and how we can make America safe and prosperous, and that's the theme of our campaign," he said.

The television ad titled "Celeb," released on Wednesday, is set to air in 11 battleground states and intercuts images of Obama on his trip to Europe last week with video of 20-something pop tarts Spears and Hilton -- both better known for their celebrity than their accomplishments.

"He's the biggest celebrity in the world, but is he ready to lead?" the voiceover asks, noting the Illinois senator's opposition to offshore oil drilling and suggesting he would raise taxes if elected.

Click here to watch McCain's ad, titled "Celeb" | Click here to watch Obama's rebuttal ad, titled "Low Road"

On Thursday, Obama responded to the ad by suggesting McCain isn't a serious candidate.

"Given the seriousness of the issues, given the fact that the decisions we make right now are going to help to determine the future of not just the next generation but perhaps generations after that, given the magnitude of our challenges when it comes to health care and energy and jobs and our foreign policy, you’d think we’d be having a serious debate," Obama said.

"But so far, all we’ve been hearing about is Paris Hilton and Britney Spears. I do have to ask my opponent, is that the best you could come up with?"

The Obama campaign, which quickly responded to the attack ad with a commercial of its own, dismissed McCain's complaints as "baloney" and "baseless" and began a fund-raising pitch built on the new Celeb ad.

"Less than 24 hours ago, the McCain campaign launched the latest and lowest in a series of misleading attack ads," the letter reads. "This Karl Rove-style ploy misleads people about Barack's energy plan and even mocks his ability to inspire voters and bring Americans back into the political process."

The McCain campaign has suggested the ad isn't an attack on Obama's character, but merely a notation of fact.

"The reality is Senator Obama is a celebrity the likes of which we haven't seen in the political arena certainly in my lifetime," said McCain campaign adviser Nicolle Wallace. "So I think you must start in every discussion by acknowledging reality and by pointing out that in this election you can't just inspire people with your words or your crowds or your images or your photos, but you must inspire them with your deeds and with your vision. And so that's the gap that we see opening up and that we're pretty excited and motivated by."

Wallace added that the Obama campaign reacted so strongly to the ad because of what she described as the underlying truth of the message.

"They went after what they obviously feel vulnerable and fussy about, and that is the insinuation that perhaps there is, one, a humility gap emerging; and, two, that on the substance they are dead wrong. ... Nobody likes a fussy man," she said.

The latest attempt to go on offense is risky for McCain, who faces the challenge of acknowledging Obama's worldwide fame and trying to depict it as a weakness rather than a strength.

“Barack Obama is the biggest celebrity in the world, comparable to Tom Cruise, Britney Spears and Paris Hilton,” Rick Davis, McCain’s campaign manager, wrote in an e-mail addressed to interested parties after throwing out a similar line on a conference call with reporters.

“Only celebrities like Barack Obama go to the gym three times a day, demand “MET-RX chocolate roasted-peanut protein bars and bottles of a hard-to-find organic brew — Black Forest Berry Honest Tea — and worry about the price of arugula,” Davis said.

Campaigning in Missouri on Wednesday, Obama said the McCain ad was the latest example of McCain's negativity -- a theme his campaign has tried to stress lately.

"He doesn't seem to have anything positive to say about me, does he?" Obama said. "You need to ask John McCain what he's for, not just what he's against."

Obama also said the link to Hilton shows Republicans are leaving no stone unturned in their attempts to tarnish him.

"You know, the latest one they've got me in an ad with Paris Hilton. (laughter) You know... never met the woman. But, but, you know, what they're gonna try to argue is that somehow I'm too risky," he said.

Hilton's spokesman Jason Moore commented, saying "Miss Hilton was neither asked, nor did she give permission, for the use of her likeness in the ad, and has no further comment." Hilton's parents reportedly donated $4,300 to McCain's campaign.

The Obama campaign ad, released hours after McCain's, shows images of the Arizona senator with President Bush and accuses McCain of practicing "the politics of the past." The campaign said it could air as soon as Thursday.

It was the second Obama ad in as many days responding to negative spots by McCain. But it was unclear how broadly the campaign intended to run it. The campaign typically identifies states where its ads air, but on Wednesday it said only that the ad would appear "in some markets."

McCain's pride in the ad did not please 23-year-old University of Wisconsin-Parkside student Wendy Chavours-Freeman, who supports Obama. Chavours-Freeman told FOX News she came to the McCain event in Racine, Wis., to "get perspective" but was unsatisfied with his answer.

Former Arkansas Gov. and Republican presidential candidate Mike Huckabee also weighed in on the controversy Thursday, saying of Obama, "It's pretty clear that he's off his game and there's nothing worse to try to be president than not being able to control the flow and the tempo of the game. If you're going to be president, you have to be able to react to things.

"And he's reacting, but whining. And the fact is, maybe his new motto should be, 'we will serve no whine before its time,'" he said.

The Associated Press contributed to this report.