Another Obama ad suggests that Mitt Romney paid no taxes in some years

President Obama’s campaign is out with its second ad suggesting that former Massachusetts governor Mitt Romney skirted taxes altogether at some points in his career.

As with a previous ad raising the same idea, this spot combines facts about Romney’s money-management history with allegations that can’t be proved — at least, unless the Republican candidate releases his tax returns.

“Did Romney pay 10 percent in taxes? Five percent? Zero?” the narrator asks. “We don’t know.” A clip plays in which Romney says he’s “happy to go back and look” at his past tax returns.

The narrator continues, “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. One of the largest tax avoidance schemes in history. Isn’t it time for Romney to come clean?”

Romney chaired the audit board of Marriott International from 1993 to 1998, as detailed in a Bloomberg article in February. During that time, according to the article, the company set up a tax shelter known as “Son of BOSS,” an acronym for “bond and option sales strategies.” Companies could avoid taxes by off-setting gains with paper losses. The IRS lost billions to these schemes and began cracking down on them as abusive in 2000; Marriott’s setup was invalidated by a federal appeals court in 2009.

Asked about Romney’s role, campaign spokeswoman Andrea Saul told Bloomberg, “For details of Marriott’s tax planning, we refer you to Marriott.”

Romney has refused to release tax returns from before 2010, leading some Democrats to speculate about what he could be hiding — and others to outright accuse him of tax dodging.

Senate Majority Leader Harry Reid (D-Nev.) has repeatedly claimed that a former Bain investor told him that Romney did not pay taxes at all for 10 years.

Obama’s campaign has not endorsed that attack, but it continues to edge close to that line.