Democratic National Committee executive director Patrick Gaspard today called Mitt Romney's tax return release "limited" and "insufficient" - insisting his party will be unrelenting in its criticism as long as the former governor remains a presidential candidate.

Gaspard - a political strategist leading the onslaught of Democratic attacks on Romney in coordination with President Obama's re-election campaign - told reporters on a conference call that producing only one year of returns leaves Americans unable to evaluate whether Romney has "financial conflicts that could cloud his judgment."

"Romney's failure to release years worth of tax returns not only defies precedent for presidential candidates… but it further weakens the central premise of Romney's candidacy," Gaspard said.

"He's asked voters to judge him on his real economy experience at Bain Capital. Yet these returns don't show how he made his mass fortune while he was there or whether he ever paid taxes on it because he's continuing to keep those returns secret, defying decades of precedent set by his own father."

Gaspard also said Romney's defense of the same tax system that allows some wealthier Americans to effectively pay a lower rate than members of the middle class is itself a glaring conflict of interest - underlining a theme that will be central to Obama's State of the Union address tonight.

"He opposes the Buffett rule to ensure that millionaires and billionaires do not pay lower tax rates than the middle class does. Now we know why," Gaspard said. "Mitt Romney pays a lower tax rate than most police and firefighters, teachers and small business owners and he doesn't want that to change."

Asked whether Democrats' scrutiny of Romney's taxes amounts to an assault on financial success, Gaspard strongly disagreed.

"We're not having a conversation about his wealth," he said.

"He's advanced tax policies that benefit the wealthiest Americans who have the benefited from the same loopholes and tax bracket that he happens to inhabit. And it's altogether right for voters in Florida and in upcoming states to ask whether or not Romney is simply advancing policies and principles that directly benefit him, create conflict of interest or a detriment to ordinary working Americans."