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As the debate intensified, Mr. Obama was undercut somewhat by a wealthy ally, billionaire investor Warren Buffett. Mr. Buffett, who supports raising taxes on people such as himself, broke with the White House by writing in an op-ed in The New York Times that everyone who makes under $500,000 should continue to pay the lower tax rates enacted under President George W. Bush.

“Let’s forget about the rich and ultrarich going on strike and stuffing their ample funds under their mattresses if — gasp — capital-gains rates and ordinary income rates are increased. The ultrarich, including me, will forever pursue investment opportunities,” Mr. Buffett wrote.

Mr. Carney said Republicans’ discussion of raising substantial revenue through closing loopholes and limiting tax deductions “is not necessarily realistic.”

“I think there’s a growing consensus … that we have to do this in a balanced way, and that revenue has to be part of it,” Mr. Carney said.