WASHINGTON — President Bush sought yesterday to convince a skeptical public that America is on the cusp of winning the war in Iraq, arguing in a speech at the Pentagon that the recent buildup of American forces has stabilized that country and “opened the door to a major strategic victory in the war on terror.”

Vice President Cheney said separately that it does not matter whether the public supports a continued American presence in Iraq, and he likened Mr. Bush’s leadership to that of Abraham Lincoln during the Civil War.

After a reporter cited polls showing that two-thirds of Americans oppose the Iraq war, Mr. Cheney responded: “So?”

“I think you cannot be blown off course by the fluctuations in the public opinion polls,” he added in an interview in Oman with ABC News. “There has in fact been fundamental change and transformation and improvement for the better.”

The confident remarks came on the fifth anniversary of the invasion of Iraq, marking a concerted effort by the administration to highlight progress at a time when most Americans remain opposed to the venture.

The anniversary prompted new attacks against Mr. Bush by Democrats and sparring among the three senators running to replace him. It also thrust Iraq back into the center of the Washington debate after it was overshadowed for months by the presidential campaign and economic turmoil.

Democratic presidential contenders Hillary Clinton of New York and Barack Obama of Illinois sharply criticized Mr. Bush for his handling of the war. Mr. Obama, speaking a day after delivering a widely watched speech on race relations, also sharpened his attacks on Mrs. Clinton and the Republican nominee, John McCain of Arizona, casting them as political opportunists who made the wrong call by voting to authorize the war.

“There is a security gap in this country — a gap between the rhetoric of those who claim to be tough on national security and the reality of growing insecurity caused by their decisions,” Mr. Obama said in Fayetteville, N.C.

While wrapping up a two-day visit to Israel, Mr. McCain echoed Mr. Bush’s message, saying that “America and our allies stand on the precipice of winning a major victory against radical Islamic extremism.”