"One in four millionaires pays a lower tax rate than millions of hardworking middle-class households," Obama told a group of business executives gathered at the White House.
He later added, "it's just plain wrong that middle-class Americans pay a higher share of their income in taxes than some millionaires and billionaires."
The so-called Buffett Rule -- to be voted on next week in the Democratic-run Senate -- would require millionaires and billionaires to pay at least the same tax rate as other Americans.
Senate Republicans are likely to block the proposal, calling it an election-year gimmick that amounts to a growth-slowing tax hike.
Republican Party Chairman Reince Priebus called it the "Buffett Ruse" and said it "won't balance the budget. It won't prevent a debt crisis. It won't help the economy. It won't get you a job."
The proposal is named for Warren Buffett, the billionaire Omaha investor who said he pays a lower tax rate than his secretary.