WASHINGTON — The White House Tuesday cranked up pressure on Republican front runner Mitt Romney to release his income tax records, as Democrats built a pre-election case against the millionaire former businessman.

Romney is facing fire from top rivals for the Republican presidential nomination over his failure to so far open his personal books, amid a furor over the millions he earned as a venture capitalist before turning to politics.

The White House stopped short of calling on Romney to publish his tax returns, but pointed out that leading past candidates, including now President Barack Obama, had revealed their personal financial records.

"It's not for us to call on someone to release his tax records, but it is an established tradition for presidential candidates to release their tax record," said White House spokesman Jay Carney.

"Then-senator Obama did release multiple years of his tax records and obviously has released his tax records, as tradition dictates, since he's been president."

Carney also noted that George W. Bush, Bill Clinton and nominees for each party for years had provided the information and suggested Romney's own father George Romney established the tradition in the 1968 presidential campaign.

In a convoluted answer at a debate in South Carolina on Monday, Mitt Romney suggested that he would eventually release his tax records, despite his refusal to do so earlier in the race towards November's general election.

"If I become our nominee, and what's happened in history is people have released them in about April of the coming year, and that's probably what I would do," Romney said.

The former Massachusetts governor has been under fire for days over his claims that his former firm Bain Capital created over 100,000 jobs on his watch, despite claims he was a "vulture capitalist" who shut down companies for profit and in fact destroyed thousands of jobs.

Some political liability for Romney may lurk in the details of his tax rate.

Earlier Tuesday, he reportedly said he paid a tax rate of around 15 percent -- in line with tax law -- as most of his earnings came from investments rather than in traditional paychecks subject to income taxes.

"It?s probably closer to the 15 percent rate than anything," Romney was quoted as saying by the New York Times.

"My last 10 years... my income comes overwhelmingly from investments made in the past, rather than ordinary income, or rather than earned annual income."

Romney, who is reportedly worth over $250 million, could fall prey to an Obama campaign narrative that the very rich should shoulder more of the burden for reviving the US economy.

Obama backs a plan to raise taxes on top earners known as the Buffett rule after billionaire investor Warren Buffett who has complained the richest Americans pay a lower rate of tax than the middle classes.