AURORA, Colo. — John McCain's presidential campaign yesterday released a television ad comparing Barack Obama to Britney Spears and Paris Hilton, suggesting the Democratic contender is little more than a vapid but widely recognized media concoction.

Obama's campaign responded with a commercial of its own, dismissing McCain's complaints as "baloney" and "baseless."

McCain's ad, titled "Celeb" and set to air in 11 states, intercuts images of Obama on his trip to Europe last week with video of twenty-something pop stars Spears and Hilton — both better known for their childish off-screen antics.

"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.

It was the latest effort by the GOP hopeful to cast Obama as a lightweight with little experience in leadership or governing.

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 today.

Kathleen Hall Jamieson, a communications professor at the University of Pennsylvania, said McCain's comparison of Obama to Spears and Hilton likely would not persuade many voters.

"The typical viewer will fail to see the analogy," she said. "Voters believe Sen. Obama is a celebrity, but not in the same sense as Britney Spears and Paris Hilton."

McCain did not mention the ad at a town-hall meeting in Colorado, but continued to attack Obama.

"The beauty of his words have attracted many people, especially among the young to his campaign," McCain told workers at Wagner Equipment, which rents and sells farm machinery. "My concern with Sen. Obama is with issues big and small. What he says and what he does are often two different things."