Democrat Barack Obama on Thursday lamented the sour tone of the presidential campaign, but insisted he and his wife have created a protective bubble around their two young daughters.

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In recent weeks, as he has drawn even with his chief rival, Hillary Clinton, she has kept up a drumbeat of criticism that Obama is too inexperienced to be elected president. One of her top advisers resigned after suggesting Obama would be vulnerable to questions about his use of illegal drugs as a teenager. A Clinton surrogate raised Obama's Muslim heritage roots in what some viewed as an unflattering way. And he and Clinton have engaged in debate about their health care plans.

"So far, I think, attempts to go negative in a way that's not policy-based have backfired on the people who have gone in that direction," Obama said during a brief interview with The Associated Press after a town hall-style meeting at an American Legion hall.

"If people are arguing about policy, that's part of politics and that's fair," said Obama, an Illinois senator.

Obama said campaign ads focused on policy, such as his spot criticizing Clinton's universal health care plan, are fair game and he will be aggressive about making such comparisons. While not pleasant, he said he can't fault his rivals for them.

"I would distinguish between ads that I would say maybe mischaracterize my positions but had to do with policy, versus personal attacks or attempts to go at my character or those things. In which case, I will answer them swiftly and truthfully if they're false and trust in the voters," Obama said.

The back-and-forth between Obama and Clinton has not affected his daughters, Obama said. The girls, Malia and Sasha, have appeared with their parents at some campaign events and were included in a holiday campaign commercial Obama released this week.

"Part of the reason that Michelle and I were comfortable doing this is our daughters are young enough. They're not watching the news," Obama said. "They're watching Nickelodeon. You know, they're 9 and 6. They're watching 'Hannah Montana."'

Earlier this month, Bill Shaheen, a co-chair of Clinton's New Hampshire campaign and husband of the state's former governor, resigned after suggesting that Obama's admitted use of illegal drugs as a teenager could be used against him if he won the Democratic presidential nomination. Clinton later denounced the comments, which she said were unauthorized, and personally apologized to Obama.

This week, former Sen. Bob Kerrey endorsed Clinton but praised Obama's Muslim roots in a way that raised eyebrows because that detail has been part of an Internet smear campaign that falsely suggests Obama is a Muslim who wants to bring jihad to the United States. Obama is a Christian. Kerrey later apologized for any unintentional he may have committed.

Kerrey's letter of apology was reprinted in The New York Times. It reads, in part:

"You are exceptionally qualified by experience and judgment to be president of the United States. I do not doubt that you would use the power of the presidency to bring peace and prosperity to as many people as possible on our fragile planet. You inspire my highest hopes for that office’s potential: That it be used as a force for good in America and the world.

"It is your capacity to inspire hope that is your greatest God given talent. Without spending a dime of tax payer’s money or changing a single law your presence in the Oval Office will send a clear and compelling message to four groups of people who will be altered for good as a consequence."

Clinton's husband, for President Bill Clinton, has also criticized Obama's resume, saying he's too young and inexperienced to be president.

But as a first-time presidential candidate, Clinton cast himself as an "agent of change" in troubled times, and faced criticism from Republicans about his youth and inexperience.

On Monday, Obama took issue with the former president.

"Here's a quote," Obama said with a smile. "'The same old experience is irrelevant. You can have the right kind of experience or the wrong kind of experience. And mine is rooted in the real lives of real people, and it will bring real results if we have the courage to change."'

"And that was Bill Clinton in 1992."