WASHINGTON — By the time the Vietnam war ended in 1975, it had become America's longest war, shadowed the legacies of four presidents, killed 58,000 Americans along with many thousands more Vietnamese, and cost the U.S. more than $660 billion in today's dollars.

By the time the bill for World War II passed the $600-billion mark, in mid-1943, the United States had driven German forces out of North Africa, devastated the Japanese fleet in the Battle of Midway, and launched the vast offensives that would liberate Europe and the South Pacific.

The Iraq war is far smaller and narrower than those conflicts, and it has not extended beyond the tenure of a single president. But its price tag is beginning to reach historic proportions, and the budgetary "burn rate" for Iraq may be greater than in some periods in past wars.

If U.S. involvement continues on the current scale, the funding for the Iraq war — combined with the conflict in Afghanistan and other foreign fronts in the war on terrorism — is projected to surpass this country's Vietnam spending next year.

And the accumulating cost is adding to resistance to President Bush's war policy in Congress as well as in public opinion, even though concern about the cost in human lives, the war's impact on America's place in the world and other such factors loom larger.

Last week, when Bush unveiled his new war plan — which included sending an additional 21,500 U.S. troops to Iraq and launching another effort to provide jobs and public services in Baghdad — the cost issue was raised by Republicans as well as Democrats.

But it had been simmering for more than a year.

Members of Congress have talked relatively little about the war's accumulating price tag because of the human costs, Rep. Zoe Lofgren (D-San Jose) said. "But certainly we're cognizant of it," she said. "When you say for what we're spending in a month in Iraq, you could fully fund and double the science budgets of the United States and come up with a viable alternative to oil, it puts it in perspective."

Even so loyal a Republican as Sen. Judd Gregg of New Hampshire, who chaired the budget committee until the Democrats took control of the Senate this year, criticized the administration's approach to war costs, calling it "without any discipline as to how much is going to be spent."

"They're gaming the system," Gregg said.

At a media briefing before Bush's speech Wednesday night, a senior administration official said the president's plan would entail $5.6 billion in military expenses and $1 billion in reconstruction and other civilian costs.

In the broad landscape of federal spending, those are not huge numbers, though $6.6 billion is more than enough to cover the budgets for all the country's national parks, national forests, historic monuments, protected wetlands and wildlife refuges for a year.

What makes the cost issue increasingly sensitive is not just questions about whether it will buy success but also the fact that the new plan's cost will add to a mountain of bills for earlier military and reconstruction efforts with what many see as little or no positive return on the investment.

Some Republicans, especially fiscal conservatives worried about the deficit, are particularly unhappy because, they say, the president and the Defense Department have refused to address the war's impact on the budget in a straightforward way.

Instead of including war costs in the regular budget, such as the one Bush will send to Congress next month, the administration has been asking Congress for emergency-spending bills that short-circuit many of the usual review procedures for appropriating funds.

"Muting and undermining the legitimacy of the congressional role in funding is, I think, undermining to some degree the commitment to the war effort itself," Gregg said.

The administration says its approach is necessary because it is unable to determine what it will need for the war in the coming fiscal year, which begins each October. Critics say that may have been true early in the war but that by now most costs are predictable far in advance.

Last year, Congress approved a provision in the annual defense authorization bill calling on the administration to change course and put its request for war funds in regular spending bills subject to full congressional review.

Said Sen. John McCain (R-Ariz.), the provision's author: "Neither the White House nor the Congress is making the tough decisions about how we are going to pay for the ongoing wars. Adding hundreds of billions of dollars that are more conveniently designated as emergency expenditures — so they do not have to be budgeted for along with other national priorities — is only making our fiscal problems that much greater."

Sen. Edward M. Kennedy (D-Mass.) proposed that Congress block Bush's new plan by withholding funds. To date, Congress has not used its power of the purse to limit Bush's prosecution of the war, partly because it doesn't want to seem to deny U.S. troops any needed support.

"If you cut off funding, you're cutting off support for the troops," said Rep. C.W. "Bill" Young of Florida, a top Republican on the House Appropriations Committee. "Whether you support the battle they're involved in or not, the vast majority of the American public is still very supportive of our troops."

During Vietnam, Congress did threaten to limit the use of the defense budget. At one point, for example, it prohibited the use of funds in Cambodia.

But Congress flexed its fiscal muscles only toward the end of the Vietnam War. Bush's war on terrorism is in its seventh year, and at a comparable stage of Vietnam, antiwar lawmakers could muster only a handful of votes for limiting funds.

From the beginning of President Lyndon B. Johnson's troop buildup in 1965 to the fall of Saigon in 1975, the United States spent the equivalent of $662 billion in 2007 dollars, according to the Congressional Research Service. The war in Iraq is harder to measure because its costs tend to be mixed up with those of the war in Afghanistan and Bush's broader global war on terrorism, says Steven Kosiak, director of budget studies at the Center for Strategic and Budgetary Assessments in Washington.

Starting with the anti-terrorism appropriation enacted a week after the attacks of Sept. 11, 2001, Kosiak figures the United States had spent $400 billion fighting terrorism through fiscal 2006, which ended Sept. 30.

For fiscal 2007, Congress has so far approved $70 billion. The president is expected to ask Congress for $100 billion more.

Even if the fighting stopped soon, which few expect, the bills would continue to accumulate as the Pentagon pushed to restore what the war had cost in troops and material.