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With economic issues at the forefront of the campaign, Mr. Romney and Mr. Obama have been trading attacks over tax policy.

Mr. Obama has cited an independent analysis that says, given the details Mr. Romney has released about lowering tax rates on the wealthy and his pledge not to raise the overall level of taxes, he will have no choice but to eliminate tax breaks that benefit the middle class.

Mr. Obama also argued that Mr. Romney’s push to reshape Medicare into a voucherlike program would force seniors to pony up an additional $6,400 for the same level of benefits — though that figure has been disputed.

Mr. Obama upped the ante further in a surprise visit Monday to the daily White House press briefing, telling reporters that voters “would rightly expect” Mr. Romney to release more of his own personal tax returns.

“I think people want to know that, you know, everybody’s been playing by the same rules including people who are seeking the highest office in the land,” he said, adding his voice to a chorus of Democrats and Republicans who say Mr. Romney, whose personal fortune is estimated as high as $250 million, should be more forthcoming about his tax history.

“If Romney intends to win, he is going to have to make the tax forms public,” the editorial board of New Hampshire’s Union Leader, the state’s largest newspaper, wrote last month. “This storm won’t go away. It will distract from his policy debate with Obama — and it will distract from Obama’s failures, providing the incumbent with the smoke screen he is attempting to create.”

Mr. Romney, though, has said he will not bow to pressure. To date, he has released a tax return from 2010 that showed his effective tax rate was 13.9 percent and a partial return for 2011 that shows he would pay a rate of 15.4 percent.

Last week, his campaign turned down an offer from the Obama campaign to stop talking about his taxes if he would release a total of five years worth of returns.

Mr. Romney said Thursday he has paid at least 13 percent of his income in taxes each year over the past decade.