RACINE, Wis. — Republican presidential candidate John McCain expressed pride Thursday in a new campaign ad that compares Barack Obama to a pair of Hollywood celebrities but defended his Democratic opponent after a voter said Obama "terrifies me."

The ad intersperses images of Obama's appearance before cheering fans in Berlin last week with clips of Britney Spears and Paris Hilton. It underscores McCain's oft-stated criticism that Obama is more a media phenomenon than someone prepared to assume the presidency but also led some Republican supporters of McCain to complain the ad was childish.

"What we are talking about here is substance and not style," McCain said, making his first comments about the ad after a voter questioned him about it. "Campaigns are tough, but I'm proud of the campaign we have run, I'm proud of the issues we have tried to address with the American people ... All I can say is we are proud of that commercial."

In a conference call with reporters, Obama campaign manager David Plouffe said McCain "kind of went into a ravine in Racine" for saying he was proud of the ad.

"We can most assuredly tell you that voters around the country do not think there's anything substantive about this latest ad, do not think it's something that John McCain should be proud of," Plouffe said.

McCain later praised his rival during the Wisconsin town-hall meeting when an 18-year-old woman criticized Obama.

"I just have to say, Obama terrifies me," she said. "I just want to say I think you should call him on every shot. Don't let him get away with a single ... we can't afford it."

"I respect and admire Sen. Obama. We just have stark differences," McCain replied, ticking through his usual list of complaints about Obama, such as his refusal to support the Bush administration's decision last year to send thousands more U.S. troops to Iraq.

With most national and battleground state polls showing McCain running slightly behind Obama, the GOP hopeful has been ratcheting up his attacks. At the same time, he has cut back on his interaction with the reporters traveling with the campaign — jettisoning his trademark looseness with the press and letting his staff and surrogates mix it up instead.

McCain took direct aim Thursday at Obama's signature pledge to bring fundamental change to Washington.

"If Sen. Obama doesn't have the strength to speak openly and directly about how he would address the serious challenges confronting America, how would he be strong enough to really change Washington?" McCain asked. "We don't need another politician in Washington who puts self-interest and political expediency ahead of problem solving."

In Wisconsin, McCain and his wife, Cindy, also had coffee with Debra Bartoshevich, a former Hillary Rodham Clinton delegate to the Democratic National Convention who switched her support to McCain after Clinton dropped out. The state party has voted to strip Bartoshevich, a nurse from Waterford, of her status as a delegate after she announced she would vote for McCain in November.