President Lyndon Johnson didn't mention Vietnam as he discussed the major issues facing the nation during the long night after John F. Kennedy was assassinated in November 1963. But by 1968, amid mounting US casualties and Pentagon requests for more troops, the unpopularity of the war forced Johnson to drop out of the race for reelection, leaving him a broken man, a former adviser recalled yesterday.

''No president can win a war when public support for that war begins to decline and evaporate," said Jack Valenti, special assistant to Johnson from 1963 to 1966. ''It's like letting a heavy body roll down a hill and, once you let go, you lose control of it."

Few people have greater empathy for the challenges facing the Bush administration in Iraq than Valenti and the other speakers who appeared yesterday at a unique conference on ''Vietnam and the Presidency" at the John F. Kennedy Library in Dorchester. As top advisers to the Vietnam-era presidents, they know firsthand about making decisions based on unreliable intelligence and the fear that failure would only embolden US enemies around the world.

But the four men -- Valenti, Henry Kissinger, Theodore Sorensen, and Alexander M. Haig Jr. -- had plenty of hard-earned lessons for Bush and other political leaders from the worst military defeat in US history, costing more than 57,000 soldiers' lives. Though the lessons depend partly on ideology, all four saw similarities between Vietnam and the war in Iraq.

''Every asset of the nation must be applied to the conflict to bring about a quick and successful outcome, or don't do it," said Haig, an adviser to presidents Johnson and Nixon who says more troops are needed to succeed in Iraq. President Bush's father had 660,000 coalition soldiers for the 1991 Gulf War invasion, more than twice as many as his son had for Iraq's initial invasion.

''Vietnam and the Presidency," the first conference sponsored jointly by all the presidential libraries, drew more than 500 people to the Kennedy Library, including some who arrived at 6 a.m. for good seats and another 1,000 watching by satellite from the nearby University of Massachusetts. Caroline Kennedy, President Kennedy's daughter, introduced the morning discussion among the four former advisers, all approaching or past 80 and still strikingly sharp in recalling events of decades ago.