President Barack Obama on Monday effectively repudiated a controversial and misleading ad, crafted by a super PAC fighting for his re-election, that essentially blamed Mitt Romney for the cancer death of the wife of a laid-off steelworker. The White House had previously resisted calls to denounce the commercial, which has run only once as a paid ad but has seized headlines and aired on news programs since its release.

"I don't think that Gov. Romney is somehow responsible for the death of the woman that was portrayed in that ad," Obama told reporters during a surprise visit to the White House briefing room.

"But keep in mind this was an ad that I did not approve, I did not produce and as far as I can tell has barely run. I think it ran once," the president said. And he accused Romney of personally embracing "patently false" attacks on his record on welfare.

Obama also defended himself from charges—notably from Romney—that he is running a negative campaign.

"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 … but we don't go out of bounds," he said.

Obama bristled at the reporter's question, which noted that his campaign has suggested that Romney either lied to the public or committed a felony with his handling of Bain Capital.

"Nobody accused Mr. Romney of being a felon," he said.

But deputy Obama campaign manager Stephanie Cutter said on a July 12 conference call with reporters that Romney either misrepresented the timing of his departure from Bain to the Securities and Exchange Commission, "which is a felony," or was "lying to the American people" about when he left the firm.

"After spending weeks refusing to denounce his super PAC's scurrilous ad against Mitt Romney, President Obama once again failed to lead," said Romney campaign spokesman Ryan Williams.

"President Obama's failure to stand up to dishonest rhetoric and attacks demonstrates yet again he's diminished the office that he holds and his record is nothing more than business as usual in Washington," Williams said.