U.S. millionaires are paying the lowest income tax rates in half a century, and the nation could not afford for this tax break to continue, U.S. President Barack Obama said on Wednesday.

Speaking at the White House, Obama again used the "Buffett Rule " to stress the importance of economic fairness to the nation's prosperity in his second straight day of campaigning for legislation that would raise the tax rate for the rich. The " Buffett Rule" is named after billionaire investor Warren Buffett, who has famously said he pays a lower tax rate than his secretary.

The wealthy should do their fair share in helping reduce the country's massive deficits, Obama said.

"At a time when the share of national income flowing to the top 1 percent of people in this country has climbed to levels we haven 't seen since the 1920s, these same folks are paying taxes at one of the lowest rates in 50 years," he charged.

"While many millionaires do pay their fair share, some take advantage of loopholes and shelters that let them get away with paying no income taxes whatsoever -- and that's all perfectly legal under the system that we currently have," added Obama who was flanked by corporate executives who agreed with the "Buffett Rule" in the briefing room.

Nearly one-quarter of all U.S. millionaires pay a lower tax rate than millions of middle-income taxpayers currently, figures from the White House showed.

The speech came a week before the Senate is scheduled to vote on a bill that would require U.S. taxpayers with an annual income of 1 million dollars or more to pay an income tax rate of at least 30 percent.