In a conference call with reporters Thursday, Douglas Holtz-Eakin, Mr. McCain’s senior economic adviser, complained about what he described as flaws in the Tax Policy Center study. He called it “an incomplete analysis” that made “some fundamentally unrealistic assumptions” about Mr. Obama’s plan, specifically in the area of taxes on small businesses, and accepted spending projections for the Democrat “that also don’t add up.”

There is “a real difference here” between Mr. McCain and Mr. Obama, Mr. Holtz-Eakin added. “The Tax Policy Center didn’t capture that,” he said.

Part of the disagreement between the candidates has to do with the semantics of what constitutes a tax increase. When Congress approved cuts in 2001 and 2003, it did so with the understanding that those benefits, which largely favor the wealthy, would expire at the end of 2010. While Mr. McCain wants to extend virtually all of the cuts permanently, Mr. Obama favors retaining those that primarily benefit people whose income is less than $250,000 a year.

“The way the Congressional Budget Office scores it, that would not be a tax increase, because their baseline assumes the increase is going to happen as part of current law,” said Chris Edwards, a tax analyst at the libertarian Cato Institute. “Bush and the Republicans would have liked the cuts to be permanent, but you need 60 votes in the Senate to make cuts permanent.”

Mr. McCain is correct when he says that Mr. Obama intends to increase the maximum tax rate on capital gains, the bulk of which fall on the wealthiest segment of the population, and that he would be less generous in offering breaks on the estate tax. But he tends to play down or ignore Mr. Obama’s proposals to eliminate taxes for retirees earning less than $50,000 a year and to give tax breaks to workers earning less than $75,000 annually.

“McCain is picking the areas where rates go up and ignoring the areas where Obama is trying to rebalance the tax code so that taxpayers would save,” said John Irons, research and policy director at the Economic Policy Institute, which is generally viewed as sympathetic to working families. Mr. Irons said that “the important thing is to look at overall impact on people” and that on this score, “the vast majority of the population, almost the entirety of the middle class, would see more from Obama than McCain.”

Economists have also criticized the methodology behind Mr. McCain’s assertion that Americans from all kinds of backgrounds could end up paying thousands of dollars more in taxes if Mr. Obama got his way. Several criticized him as apparently basing his claim on an average figure in which, as Mr. Irons said, “Bill Gates is mixed with you and me, and everything gets skewed.”