(Charles Dharapak/AP Photo)

The Obama campaign is renewing calls on presumptive Republican presidential nominee Mitt Romney to release more of his tax returns, saying a fuller disclosure is the only way to resolve questions about his secretive offshore finances, which Democrats have been scrutinizing for the past week.

"These issues are important because voters can't make judgments about Mitt Romney's motivations on critical policy matters if this information is kept hidden," deputy Obama campaign manager Stephanie Cutter said in a statement today. "If there's nothing to hide, why not clear it all up and release multiple years of returns like every candidate has done for decades, including Mitt Romney's own father?"

In January, Romney released copies of his 2010 tax return after immense public pressure from his GOP primary rivals and Democrats. But he stopped short of releasing the 12 years worth of returns that his father did when running for president and as President Obama has done. Public release of a candidate's tax returns is not required by law but has been precedent set by candidates in both parties.

Democrats allege that Romney's lack of transparency suggests that he has been "betting against America" through investments abroad and benefiting from special tax loopholes through a Swiss bank account, holdings in the Cayman Islands and ownership of a Bermuda corporation.

In a new campaign web video, Obama campaign spokesman Ben LaBolt continued to pepper Romney with questions about his portfolio, aiming to boost the aura of suspicion around the Republican's financial activities even though he does not present any evidence of wrongdoing.

Get more pure politics at ABC News.com/Politics and a lighter take on the news at OTUSNews.com

"The Romney campaign said that when we pointed out these simple facts we're distorting reality and launching false attacks. But they never said what was false about them," said LaBolt. "Mitt Romney could be the first president in history to stash millions offshore."

The Romney campaign has long insisted that the former governor and corporate buyout specialist has complied with all federal laws and financial regulations, and notes that no direct evidence has been presented to suggest otherwise. Aides say the Democratic attacks amount to character assassination of Romney, criticizing his financial success in an effort to promote class warfare.

"He hasn't paid a penny less in taxes by virtue of where these funds are domiciled," wrote Romney adviser Kevin Madden of the financial questions in a letter read on "Fox News Sunday." "His liability is exactly the same as if he held them directly in the U.S. As a U.S. citizen, he is accountable for U.S. taxes. Some investments in foreign countries can be tax havens. But Mitt Romney does not hold any such investments."

Romney supporter and Mississippi Gov. Haley Barbour also defended Romney in an appearance on "Face the Nation," calling the Republican candidate "a guy who has a spectacular career and record, enormously generous, has given away millions and millions of dollars that he made, not of the government's money, but of money he made."

Still, the Obama campaign shows no signs it will let up from raising questions about Romney's finances - no matter if they were technically legal - in an effort to discredit him as an acceptable alternative to President Obama.

"He's asking voters to take him at his word, since Mitt Romney is defying the long-time precedent of presidential nominees releasing multiple years of tax returns," Cutter said. "But, if he didn't receive any tax advantage, what was the purpose of creating shell corporations and investing so much of his money in tax havens in the first place? And, why did he work so hard to keep the existence of these accounts secret for so many years?"

This post has been updated.