As part of a weeklong campaign around the Buffett Rule, President Obama's re-election team is making Mitt Romney the face of income tax inequality.

On a conference call with reporters Monday, top Obama surrogates blasted the Republican candidate for keeping years of tax returns secret, using offshore bank accounts for some investments, and enjoying a lower effective tax rate than most middle-income Americans.

"Romney supports tax policies that reward people like him, and now he's trying to obscure just how much he would benefit, by hiding his own financial records," said Obama campaign manager Jim Messina. 

"Our message to Mitt is simple: if you don't have anything to hide, release your taxes just like every other candidate for president does," he said.

Romney has released his 2010 and 2011 income tax returns but fewer from previous years than any of his recent predecessors. The documents show he earned roughly $42 million over the two year period and paid $6 million in income taxes - or an effective tax rate of 14 percent.

The Buffett Rule, named after billionaire investor Warren Buffett and backed by Obama, would ensure that Americans earning $1 million or more in adjusted gross income pay at least 30 percent in taxes.

While legislative versions of the Rule are unlikely to pass Congress, Democrats hope their campaign on the issue will galvanize opposition to Romney, the likely GOP presidential nominee.

"People think that the system is rigged against them," said Rep. Tammy Baldwin, D-Wisc., who's sponsoring a bill to enact the rule. "They see people like Mitt Romney paying 13.9 percent and hear stories about Warren Buffett and others and think there are two systems.  Middle class families are taking it on the chin right now, and they don't see others doing their fair share."

Sen. Dick Durbin, D-Ill., who's also sponsored Buffett Rule-inspired legislation, said the bigger questions for Romney surround his transparency about his wealth.

"I disclosed my tax returns…It's time to ask the question why he's drawn the line at two years," Durbin said. "And when is the last time a presidential candidate for the U.S. had a Swiss bank account? The answer is never."

Romney has said his investment practices have complied with all U.S. laws and that his financial disclosure reports coupled with the voluntary relase of two years worth of tax returns should provide ample insight into his finances. His recent earnings were primarily derived from investment income, which are taxed at a lower rate than wages.

President Obama "wants to raise taxes on millions more [Americans] by taxing small businesses and job creators," said Gail Gitcho the Romney campaign's communications director of president's tax plan. "We appreciate the Obama campaign reinforcing Mitt Romney's platform of lowering tax rates across the board in order to jumpstart this bad Obama economy."

But Messina said Romney's opposition to the Buffett Rule and support for the Bush-era tax cuts on individuals earning $200,000 or more will be a losing message in November.

"Mitt Romney's tax plan doesn't ask millionaires and billionaires to pay their fair share. It just doesn't," Messina said.   "In fact, it doesn't just the opposite. It gives every millionaire a $250,000 tax cut which is over 1000 times greater than the tax cut he'd give the average middle class taxpayer," he said.