Obama ad: Did Romney pay zero percent in taxes?

A new ad from the Obama campaign suggests that Mitt Romney may have paid a zero percent income tax rate.

The spot opens on an interview with the presumptive Republican presidential nominee in which he is asked if he has ever paid a lower rate than the 13.9 percent income tax rate he paid in 2010. Romney responds that he would be "happy to go back and look."

"Did Romney pay 10 percent in taxes? Five percent? Zero?" a narrator asks in the spot. "We don't know."

Romney has released only his 2010 tax returns and an estimate of his 2011 returns. Democrats have called on him to release returns from previous years, with some suggesting(without evidence) that he is not doing so because he did not pay taxes in some of those years.

After raising the tax issue, the spot shifts focus.

"But we do know that Romney personally approved over $70 million in fictional losses to the IRS as part of the notorious 'Son of Boss' tax scandal," the narrator says. "One of the largest tax avoidance schemes in history. Isn't it time for Romney to come clean?"

A February Bloomberg article reported that Romney chaired Marriott International's audit board for five years in the 1990s, when the company reportedly set up the "Son of BOSS" tax shelter. It was one of the company's "complex tax-avoidance maneuvers" that angered the Internal Revenue Service, which invalidated the maneuver in 2009, calling it "fictitious" and a "scheme."

The ad is the latest in a week of harsh and unfair attacks from both sides. The Romney campaign released two spots attacking the president on welfare and an alleged "war on religion," while a super PAC backing the president suggested Romney was responsible for the death of the wife of a steelworker.

The Romney campaign called the new ad "another dishonorable and dishonest attack," though it did not directly dispute its substance in a statement released to the press.