The ad highlights an exchange from a Romney interview with ABC News in which the GOP candidate was asked if there were years in which his effective tax rate fell below 13.9 percent. Romney said he did not know, but would be "happy to go back and look."
After playing the clip of Romney answering the question, onscreen text asks, "When will Romney come clean?
Romney paid an effective rate of 13.9 percent on $21.6 million of income in 2010, the only full year of tax returns he has provided as a candidate. Democrats have criticized that number, arguing that it illustrates loopholes in the tax code that allow wealthy investors to pay a lower percentage.
Democrats have pushed for the presumptive nominee to release more records after a report in Vanity Fair detailed his offshore holdings.
The Romney campaign has repeatedly insisted that it will not turn over more returns, saying it believes doing so would simply provide Democrats more ammunition for attack ads.
Romney himself emphasized in the ABC interview that he was in full compliance with the law on his tax returns.
“From time to time I’ve been audited — as it happens, I think, to other citizens as well — and the accounting firm which prepares my taxes has done a very thorough and complete job, pay[ing] taxes as legally due,” said Romney.
“I don't pay more than are legally due, and frankly, if I had paid more than are legally due I don't think I'd be qualified to become president. I'd think people would want me to follow the law and pay only what the tax code requires,” he added.
The Romney campaign on Tuesday accused President Obama of waging a "negative campaign."
“President Obama can’t talk about his record, and he has no new ideas to make things better," said Romney spokeswoman Amanda Henneberg. "Instead, this negative campaign is the best he’s got.”