Some conservative economists say that increased deficits in the short run are an acceptable tradeoff for tax cuts that they say will promote economic growth in the long run. And many liberal economists say that some of the Democratic spending proposals, like addressing the affordability of health care or improving education, are long-overdue investments that pay off handsomely even if they entail more red ink.

Mr. Obama and Mrs. Clinton have acknowledged that their various new programs would be costly but have outlined how to pay for them. But some fiscal monitors say they may be relying on overly rosy projections of how much savings their proposals would actually yield.

Mrs. Clinton has calculated that her universal health care plan would cost about $110 billion a year, while Mr. Obama’s somewhat more modest proposal would cost up to $65 billion annually, his advisers say. Both candidates have also talked of new government incentives and investment to encourage the development of alternative sources of energy, which would cost about $15 billion a year.

The Democratic candidates have suggested that they could finance these and other programs by allowing parts of the Bush tax cuts to expire. That, however, ignores projections of the Congressional Budget Office, which has already assigned those savings to deficit reduction.

In other words, unlike Mr. McCain, both Democrats say they would revoke the Bush tax cuts for the wealthy. “At a time of war and economic hardship, the last thing we need is a permanent tax cut for Americans who don’t need them and weren’t even asking for them,” Mr. Obama said.

But they would retain those reductions meant to benefit poor and “middle-class” families, which they defined as the 97 percent or so of the population that lives on less than $250,000 a year, and they would count the estimated $50 billion generated by higher taxes on the wealthy as new revenue.

“Remember, you can only use this money once,” said Mr. Bixby of the Concord Coalition, “and with all the Bush tax cuts scheduled to expire, that money is already scheduled to come into the Treasury. But on the campaign trail, this has become a source of new spending.”