The administration wants such a tax to replace the alternative minimum tax, which was created decades ago to make sure the richest taxpayers with plentiful deductions and credits did not avoid income taxes, but which now hits millions of Americans who are considered upper middle class. Mr. Obama has said that many average Americans could see a tax cut if the system is overhauled, since ending many tax breaks would allow for lower rates while raising more revenues from the wealthiest.

The millionaires’ tax is among several changes Mr. Obama will propose in urging Congress to overhaul the federal income tax code next year, both to raise revenues for reducing deficits and to make the tax system simpler and fairer, said the administration officials, who agreed to speak in advance of the president’s announcement on the condition of anonymity.

The millionaires’ rate would affect only 0.3 percent of taxpayers, they said. That would be fewer than 450,000; 144 million returns were filed for 2010.

Mr. Obama’s proposal comes a month after Mr. Buffett began reviving his longstanding objection that he and “my megarich friends” pay a significantly lower percentage of their income in federal taxes — income and payroll taxes — than everyone else, thanks to the tax code’s favoritism toward the rich, and especially toward investors like him.

“My friends and I have been coddled long enough by a billionaire-friendly Congress,” he wrote in an opinion article in The New York Times, a complaint he has repeated in talks and media interviews since. “It’s time for our government to get serious about shared sacrifice.”

Mr. Obama has been citing Mr. Buffett as he promotes his $447 billion job-creation plan. He proposes to offset the cost of that plan and reduce future budget deficits through higher taxes on the wealthy and on corporations after 2013, when the economy will presumably be healthier.

Mr. Obama’s proposed Buffett Rule puts a new spin on that pitch, as he tries to put Republicans in Congress and in the presidential race on the defensive for their rigid stand against higher taxes.