Mr. Bush said in his speech on Thursday that the Defense Department budget today represented slightly more than 4 percent of gross domestic product, compared with more than 6 percent in some years of the Reagan administration and as much as 13 percent in 1952-3, when the United States was engaged in the war in Korea.

Image Senator John McCain, Republican of Arizona, says success in Iraq justifies any cost. Credit... Justin Lane/European Pressephoto Agency

But the war in Iraq is largely being paid for off the books, with emergency and supplemental spending rather than from the Pentagon’s operating budget, so Mr. Bush’s figures are a low estimate of the relative cost of the war, analysts have observed. And growing entitlement costs today make such comparisons with previous eras questionable.

“Even if you assume the war costs will phase down, either quickly or slowly, we are still on an unsustainable track under current budget policy,” said James R. Horney, director of federal fiscal policy at the Center on Budget and Policy Priorities in Washington, a liberal research group. “Before the war started and without any of the new ideas of the candidates, we’re in a situation where we have to reduce spending and reduce the rate of growth of Medicare and Medicaid and, to a lesser extent, Social Security. So the costs of the war have to be seen in that context.”

Douglas Holtz-Eakin, Mr. McCain’s chief economic adviser and a former director of the Congressional Budget Office, said the benefits of success in Iraq dwarfed the $150 billion annual cost. He also said that if the war and the personal and corporate tax cuts that Mr. McCain advocated added to the federal deficit and debt, so be it.

“I would like the next president not to talk about deficit reduction,” Mr. Holtz-Eakin said at a symposium sponsored by the Committee for a Responsible Federal Budget. “The next president should talk about what’s good for American families  education, health care at reasonable costs, pensions that are secure, opening our borders to trade. If we can take care of that, we can take care of the budget.”

Mr. McCain has said he plans to pay for tax cuts and modernizing the military by eliminating earmarks and wasteful spending from the federal budget. Both Senators Clinton and Obama would allow the Bush administration’s personal income tax cuts to expire in 2011 and are proposing new levies on wealthy individuals, oil companies and other businesses to help pay for expansive and expensive new government programs.

While both promise to remove troops from Iraq, Mrs. Clinton does not expect any big savings that could be used to finance her domestic initiatives. Mr. Obama does expect the winding down of the Iraq war to ultimately produce a “peace dividend” that can be used for other things.