WASHINGTON — President Bush, opening a new theme in support of sticking with the battle of Iraq, will today bluntly warn the Democrats against committing the errors of Vietnam — where America’s withdrawal precipitated a communist-led bloodbath.

In Kansas City, before the annual convention of the Veterans of Foreign Wars, Mr. Bush will deliver a speech that embraces the one war most historians say America lost. Recalling the arguments of those that sought to end the war for Saigon, the speech prepared for Mr. Bush will warn, “Then as now, people argued that the real problem was America’s presence and that if we would just withdraw, the killing would end,” according to excerpts of the speech released last night by the White House.

After quoting one anti-war senator at the time who asked what difference it would make which side won the war for Vietnam, the president will conclude: “One unmistakable legacy of Vietnam is that the price of America’s withdrawal was paid by millions of innocent citizens whose agonies would add to our vocabulary new terms like ‘boat people,’ ‘re-education camps,’ and ‘killing fields.'”

The speech also will refer to World War II, saying, “The question now before us comes down to this: Will today’s generation of Americans resist the deceptive allure of retreat — and do in the Middle East what veterans in this room did in Asia?”

The stakes for Iraq and the Bush presidency could not be higher. In July, a group of Republican senators led by Senator Warner of Virginia hinted that come September, they would need to see a change in strategy from the current surge. Democrats in May nearly succeeded in attaching a firm withdrawal date to the temporary bill that will come up again next month to fund the wars in Iraq and Afghanistan.

But the tide appears to be turning. In the last three weeks, leading Democrats, including the second-ranking member of that party in the Senate, Richard Durbin of Illinois, and the fiercely anti-Bush chairman of the Senate Armed Services Committee, Carl Levin of Michigan, have conceded after visits to Iraq that General David Petraeus has made military progress. This view has been echoed by a group of more moderate Democrats known as the “blue dogs” in the House, challenging the perception that the leadership of the party will be able to put together majorities, let alone veto-proof majorities, for withdrawal proposals.

Nonetheless, some Democratic leaders are still fighting for an exit from Iraq. In a press release sent to reporters last night, the Senate majority leader, Harry Reid of Nevada, said the president’s comparison was “off the mark.”

“President Bush’s attempt to compare the war in Iraq to past military conflicts in East Asia ignores the fundamental difference between the two,” Mr. Reid said. “Our nation was misled by the Bush administration in an effort to gain support for the invasion of Iraq under false pretenses, leading to one of the worst foreign policy blunders in our history. While the president continues to stay the course with his failed strategy in Iraq, paid for by the taxpayers, American lives are being lost and there is still no political solution within the Iraqi government.”

The fate of the war will be riding on General Petraeus and America’s ambassador in Baghdad, Ryan Crocker, who will give both public and closed-door testimony to Congress next month. On the optimistic side, the general will discuss the composition of the new salvation fronts across Iraq that are taking up arms against both Al Qaeda and the Shiite militia affiliated with Moqtada al-Sadr. The general will also point to strides made by the Iraqi security services, a position that may get him into trouble, considering his last assessments of the Iraqi military during the 2004 elections proved to be too optimistic.

On the political side, Mr. Crocker is not expected to report much progress toward reconciliation. Two of the leading Shiite parties have signed an agreement to form a ruling coalition with the two major Kurdish parties in an effort to blunt the near collapse of the government this month following the walkout of fundamentalist Sunni parliamentarians and of Shiite lawmakers loyal to Mr. Sadr.

Mr. Bush yesterday in Quebec lowered expectations for any progress on the political side and expressed some frustration with Prime Minister al-Maliki. “The fundamental question is, will the government respond to the demands of the people,” he said. “And if the government doesn’t … respond to the demands of the people, they will replace the government. That’s up to the Iraqis to make that decision, not American politicians.”

The speech today will place America’s war in Iraq in the context of America’s struggle against the Nazis, imperial Japan, and the Soviets. It says that “the fruit of American sacrifice and perseverance in Asia is a freer, more prosperous, and stable continent — whose people want to live in peace with America, not attack America.”

In another section, the speech echoes the confident tone Mr. Bush took following September 11, 2001, when he compared the Islamic fanaticism of Al Qaeda to that of the Nazis. “Today the violent Islamic extremists who fight us in Iraq are as certain of their cause as the Nazis, Imperial Japanese, and Soviet Communists were of theirs,” it says. “And they are destined for the same fate. The greatest weapon in the arsenal of democracy is the desire for liberty written into the human heart by our Creator.”

The president also reaffirms his goal of a functioning democracy in Iraq, a significant declaration in light of recent reports that the CIA, at least, favored an alternative to Mr. Maliki in a former prime minister, Iyad Allawi. “So long as we remain true to our ideals, we will defeat the extremists in Iraq and help that country’s people stand up a functioning democracy in the heart of the Middle East,” the excerpts from the speech say.

The location of the speech today is also significant. The Veterans of Foreign Wars convention was the place Vice President Cheney picked in August 2002 to telegraph many of the arguments used in the runup to the Iraq war eight months later. On Monday, Senator Clinton conceded that the surge was “working,” but added that the change in tactics in Iraq was “years too late” and that America should not prepare to fight old wars but prepare to fight new ones.

An aide to Mr. Bush, Edward Gillespie, said the president would follow up today’s speech with another address on August 28 at the American Legion convention in Reno, Nev., “in which he will put Iraq in the regional context of the Middle East.”