Washington (CNN) - Senate Democrats formally unveiled legislation Wednesday to ensure that all millionaires would pay a minimum federal tax of 30 percent.

The legislation comes as the relatively low tax rate for some high earners –like investor Warren Buffett and GOP presidential candidate Mitt Romney– takes center stage in both the policy and political arenas in Washington.

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President Obama and congressional Democrats have made enactment of the "Buffett Rule," which they say would force high earners pay a higher tax rate than their secretaries, a central part of their re-election messages. They argue it's both a matter of fairness and the best way to reduce the staggering deficit.

Buffett supports the Democrats' legislation.

"These middle class families are really struggling to get by and then they find out that some people with really extremely high incomes are actually paying lower all-in federal tax rate than they are. That's, to them, just not common sense," said Sen. Sheldon Whitehouse, D-Rhode Island, and author of the bill. "Americans deserve a straight deal and right now they are not getting one from our tax system."