"I don't think that Gov. Romney is somehow responsible for the death of the woman that was portrayed in that ad," Obama said in an impromptu appearance in the White House briefing room.
"But keep in mind this is an ad that I didn't approve, I did not produce, and as far as I can tell, has barely run. I think it ran once," he said.
The ad -- which received extensive play as the subject of news reports and cable show segments -- suggested the presumptive Republican presidential nominee's business dealings contributed to a woman's death because she lost her health insurance.
In the ad, produced by Priorities USA Action, an independent expenditure PAC that supports Obama's re-election, former steelworker Joe Soptic recounts how he lost his family health benefits when Bain Capital, a company Romney headed, shut down his steel plant in 2001. His wife died of cancer five years later.
Obama said the ad is far different from a Romney ad campaign that accuses the president of gutting welfare's work requirement, "which every single person here who's looked at it says is patently false."
Many fact checkers have corroborated Obama's position on the Romney campaign's welfare ad, The Wall Street Journal said Tuesday.
Obama defended his re-election campaign's tone, saying, "If you look at the overall trajectory of our campaign and the ads that I've approved and are produced by my campaign, you'll see that we point out sharp differences between the candidates, but we don't go out of bounds."
Concerning his call for Romney to release more than two years of income taxes -- a call the Romney campaign has suggested is beyond reasonable limits -- Obama said "that's a precedent that was set decades ago, including by Gov. Romney's father. And for us to say that it makes sense to release your tax returns, as I did, as John McCain did, as Bill Clinton did, as the two President Bushes did, I don't think is in any way out of bounds."
Romney, campaigning in New Hampshire Monday, said the Obama campaign's approach has been "sad and disappointing."
"It seems that the first victim of an Obama campaign is the truth," he said.
"When I became the presumptive nominee, the president called me and congratulated me on becoming the presumptive nominee, and said that America deserves an honest debate about the future course of the country, and I agree," Romney said.
"I'm waiting to hear him begin that, because all we've heard so far is one attack after the other," Romney said.
Romney contrasted his economic policies with Obama's, promising not to raise taxes on any American.
"I will not raise taxes on anybody. I don't want to raise taxes on the American people," he said, contrasting himself to Obama, who he said "proposed raising the [top] tax rate from 35 percent to 40 percent."
The Obama campaign has argued that Romney's call for cutting taxes across the board will necessarily raise the tax burden on middle-income Americans to balance the budget, an analysis the Romney campaign disputes.
"I will not raise taxes on the American people, I will not raise taxes on middle-income Americans," Romney said in Goffstown, N.H.
"Mr. President, stop saying something that's not the truth," Romney said to applause.
Obama Monday repeated his call for Congress to extend the Bush-era tax cuts on income under $250,000 a year for couples and a refinancing bill to help homeowners. He asked lawmakers to pass a continuing resolution on the budget to avoid an election-year government shutdown.
A continuing resolution funds government agencies if a formal appropriations bill has not been signed into law by the end of the fiscal year, Sept. 30.