Mitt Romney said Sunday that he doesn't know if there was a year for which his effective tax rate fell below 13.9 percent, but said he would be "happy to go back and look."

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.

But Romney emphasized in an interview Sunday with ABC News that he had "paid all the taxes required by law."

“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.

Pressed by ABC's David Muir as to whether he would actually go back and look at what his effective tax rate had been in previous years, Romney was evasive.

"I haven't looked at the tax rate paid year by year. I know that I pay a very substantial amount of taxes and [have] every year since the beginning of my career, so far as I can recall," Romney said.

The exchange came as pressure from the Obama campaign and even some Republicans has intensified for Romney to turn over his tax returns. Romney has said previously that he disclosed all that was required by law, and was reluctant to do more because it could give Democrats ammunition for attack ads.

On Sunday, the Obama campaign again called on Romney to provide prior years' returns.

"Amidst questions about whether he invested in foreign tax havens and offshore accounts to avoid paying U.S. taxes or hedge against the dollar, it's critical for him to make public more information about his personal finances," said Obama spokesman Danny Kanner in an email. "Since he is 'happy' to go back and look at his previous returns, he should release them for the American people to see."