Jae C. Hong/AP Photo

COLORADO SPRINGS - As he decries the negativity of campaign ads while on the stump in battleground states, President Obama should take a close look in the mirror. His campaign Thursday continued its vigorous pace of attacks on rival Mitt Romney, with a new TV ad alleging the presumptive GOP presidential nominee once participated in "one of the largest tax avoidance schemes in history."

"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?"

The ad refers to Romney's role as chairman of the audit committee for hotel giant Marriott International for six years in the 1990s. During that time, the company took advantage of complicated tax avoidance measures, including the so-called "Son of Boss" tax shelter which was later challenged successfully by the Justice Department and IRS. Romney, as an auditor, presumably knew about the practice.

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The Obama campaign says the ad, which will air in Virginia, North Carolina, Florida and Ohio, fairly illustrates a pattern of behavior by Romney to minimize tax liabilities in his personal life and in business dealings.

"These questions are especially relevant given voters' heightened attention in this election to the fate of their own tax rates, and the central role tax reform will play in the next administration," said Obama campaign spokesman Ben LaBolt in a memo to reporters.

The Romney campaign says the charge that the candidate "personally" oversaw Marriott's tax avoidance practices and had a key role in "scandal" is overblown - both "dishonorable and dishonest," said spokeswoman Amanda Henneberg.

Republicans also point out that the Obama ad's primary sourcing for its central allegation is a CNN.com op-ed published Wednesday by two major Democratic donors, "tax experts" Peter Canellos and Edward Kleinbard.

Canellos and Kleinbard have participated in press conference calls hosted by the Democratic National Committee to attack Romney, and each has donated significant sums to Democrats in election campaigns, Federal Election Commission records show.

"In 2008, candidate Obama said 'you make a big election about small things' when you don't have a record to run on. Since President Obama can't run on record unemployment, falling incomes, and massive debt, he has decided to run a dirty campaign that is an affront to everything he claims to stand for," Henneberg said.