* Biden says voters face 'stark choice' over tax policy
* Romney aide says Obama plans to stifle free enterprise
WASHINGTON, April 12 (Reuters) - U.S. Vice President Joe Biden took direct aim at probable Republican presidential nominee Mitt Romney on Thursday, accusing him of promoting what amounts to a "Romney Rule" of tax protection for the rich.
Biden's campaign speech in New Hampshire was part of a stepped-up White House effort to paint President Barack Obama's wealthy Republican challenger as out of touch with the struggles of middle-class Americans.
Both campaigns have moved quickly to frame the choice for voters after Romney's chief rival, conservative Rick Santorum, dropped out on Tuesday, giving the former Massachusetts governor a clear path to the Republican nomination to challenge Obama in the November election.
The Democratic president has spent much of this week touting the "Buffett Rule," a plan to ensure that millionaires like former executive Romney pay at least 30 percent income tax. The proposal has almost no chance of overcoming Republican opposition in Congress.
Biden, in the latest in a series of message-oriented campaign appearances, called it a "stark choice" for voters.
"The Buffett Rule says that multimillionaires should pay at least the same percentage of their income in taxes as middle-class families do," he said in Exeter, New Hampshire. He spoke from a podium with the slogan "Economic Fairness For The Middle Class" written on it.
"The Romney Rule says the very wealthy should keep the tax cuts and loopholes they have, and get an additional, new tax cut every year that is worth more than what the average middle-class family makes in an entire year," he said.
Romney's campaign struck back even before Biden spoke, accusing Obama of "transparent political gimmicks to try and distract Americans from their failure to control spending and put Americans back to work."
"Instead of the 'Buffett Rule,' the real issue is the 'Obama Rule,' which is President Obama's plan to raise taxes on American families and small businesses to grow government and stifle free enterprise," Romney campaign spokesman Andrea Saul said.
The Senate will take up the Buffett Rule, named for billionaire investor and Obama supporter Warren Buffett, next week. Obama's aides see this as a chance to hammer home accusations that Romney himself does not pay his fair share of taxes.
The Obama campaign has placed a digital calculator on its website allowing Americans to see how their tax rate compares to the 13.9 percent rate Romney paid in 2010. He paid that low rate because most of his income was from investments rather than wages, which are taxed at a higher rate.
Romney, a former head of a private equity firm who has touted his business background, opposes the Buffet Rule and has said Obama is trying to change the subject from his poor handling of the economy.
Romney wants Bush-era tax cuts for wealthier Americans that expire at the end of the year to be extended, while Obama wants to let them lapse.