Leading members of the US Democratic Party have criticised President George W Bush's speech in which he defended his Iraq policy.
Mr Bush said a military retreat could trigger the kind of upheaval seen after US forces left Vietnam.
The leader of the Senate, Harry Reid, dismissed the comparison and said the decision to invade Iraq was one of the worst blunders in US history.
Senator John Kerry called the speech irresponsible and ignorant.
Both urged the president to change course immediately.
In his address to war veterans in Missouri, Mr Bush also expressed his support for the Iraqi Prime Minister, Nouri al-Maliki, after earlier criticising his government for lack of political progress.
The president called the Iraqi leader a "good man with a difficult job".
A progress report is due in mid-September from General David Petraeus, the top American commander in Iraq, on the effectiveness of the recent surge in US troop numbers in tackling the violence there.
'Price paid by innocents'
In his address, President Bush recalled the US withdrawal from Vietnam.
"The price of America's withdrawal was paid by millions of innocent citizens," he said, adding that the Vietnam War had taught the need for US patience over Iraq.
Mr Bush compared current calls for withdrawal from Iraq with what happened at the end of the Vietnam War in 1975.
"Many argued that if we pulled out, there would be no consequences for the Vietnamese people. The world would learn just how costly these misimpressions would be.
"Three decades later, there is a legitimate debate about how we got into the Vietnam War and how we left.
"Whatever your position in that debate, one unmistakable legacy of Vietnam is that the price of America's withdrawal was paid by millions of innocent citizens," Mr Bush said, mentioning reprisals against US allies in Vietnam, the displacement of Vietnamese refugees and the massacres in Cambodia under Pol Pot's Khmer Rouge.
He warned that in Iraq there was the added danger that a US withdrawal would signal victory for al-Qaeda, emboldening its leaders and drawing in new recruits.
Senate leader Mr Reid responded: "Today our soldiers remain caught in the middle of a civil war and the president's strategy is still failing to deliver the political solution necessary for Iraq's stability.
"A change of course in Iraq is long overdue, and Congress will continue to fight for that change in the coming weeks."
Mr Bush spoke of the massacres under Cambodia's Khmer Rouge
And Senator Kerry, a decorated Vietnam War veteran who lost to Mr Bush in the 2004 presidential race, said he was not surprised that Mr Bush had sought to "oversimplify the differences and overlook the tragic similarities" between Vietnam and Iraq.
He added: "If the president wants to heed the lessons of Vietnam, he should change course and change course now."
BBC world affairs correspondent Nick Childs says Mr Bush's speech will fuel the controversy over whether he is drawing the right or wrong lessons from history.
Opinion polls suggest that many Americans clearly do not see the stakes or the struggle in the same way, our correspondent says.