WASHINGTON — Barack Obama says John McCain's plan to balance the budget doesn't add up. Easy for him to say: It's not a goal he's even trying to reach.

Not only does Obama say he won't eliminate the deficit in his first term, as McCain aims to do, he frankly says he's not sure he'd bring it down at all in four years, considering his own spending plans.

"I do not make a promise that we can reduce it by 2013 because I think it is important for us to make some critical investments right now in America's families," Obama told reporters this week when asked if he'd match McCain's pledge.

So what is more important in tough economic times? For the government to spend more to help hard-hit Americans or to eliminate a deficit that can lead to higher borrowing costs and slow the economy?

Whether it's McCain the Republican or Obama the Democrat in the White House next year, the new president is likely to inherit an annual federal deficit exceeding $400 billion. Budget watchers say it's hard to figure what either candidate's impact will be since neither is offering full details.

"I think both candidates need to put some numbers on these things so we can get a sense of how realistic they are," said Robert Bixby of the Concord Coalition, a budget watchdog group.

Obama's criticism of McCain's pledge as "overly ambitious" is backed up by fiscal experts.

"Hypothetically it's possible to get to a balanced budget by 2013, but not under the policies that McCain has proposed," Bixby said. "The policies he would propose would actually add to the deficit when you take them all together."

Jim Horney, director of federal fiscal policy at the Center on Budget and Policy Priorities, said the center's analysis found McCain would have to cut $400 billion worth of federal programs to balance the budget by 2013.

"It seems unlikely, particularly given that Senator McCain has not been willing to be specific about what programs he would cut," Horney said. "And even if he were willing to, there's a real question of whether the Congress or the public would go along with the kinds of cuts required to balance the budget, assuming the tax policies that he's proposing."

McCain says he would reduce spending by slowing the growth of Social Security, Medicare and Medicaid, but he hasn't said how he would try to change the benefits. He also says he would slow the government's spending growth and stop spending on lawmakers' special earmarked projects. But earmarks accounted for just $17 billion of the $2.9 trillion budget this year.

Obama plans to raise $100 billion annually by increasing taxes on Americans making more than $250,000 a year, money that would be used to pay for healthcare and tax cuts for low- and middle-income earners. He has said ending the Iraq war would generate about $80 billion a year, but he's not using that to pay down the deficit either. Instead, he said that money could help pay for more government programs like healthcare, education, housing and public safety.

"The problem there is that the Democrats have been very critical of President Bush for spending hundreds of billions of dollars on the war without paying for it and running up the deficit," Bixby said. "But in effect what Obama is saying is, 'I'm going to spend the same amount of money. I'm just going to spend it on something else.' "

Obama's economic policy director, Jason Furman, said the campaign will release a formal budget later that explains his spending in greater detail. Furman added that "while we would like to balance the budget, we cannot say exactly when that will happen given the need to make important investments and the large amount of uncertainty in the economy."

Bixby said even if Obama and McCain have ways to pay for their spending plans, the United States still has an unsustainable budget outlook because of rising costs for Social Security, Medicare and Medicaid.

"It's kind of like somebody building a porch on a house they can't afford," he said. "This is the part of the budget that's poised to explode over the next several years. And we really can't afford all of these new initiatives that McCain and Obama are talking about because we haven't figured out how to pay for what's already on the table."

Nedra Pickler has covered presidential politics for The Associated Press since 2002.