Lyndon Johnson knew that Walter Cronkite's words marked a defining moment.
"It seems now more certain than ever," the CBS anchor said on air in 1968, "that the bloody experience of Vietnam is to end in a stalemate."
President Johnson's reaction has been widely quoted. "That's it," he told an aide. "If I've lost Cronkite, I've lost middle America."
A few weeks later, Johnson canceled his re-election bid.
Over time, there has been disagreement about whether Cronkite's comments really did drive poll numbers. But an anti-war movement that was gathering steam at the time gained in "Uncle Walter" a highly visible and credible messenger.
At present, a majority of Americans have come to believe that the war in Iraq wasn't worth fighting. A majority also disapproves of the way President Bush is managing it. Anti-war rallies around the country are beginning to become sizable, attracting both passionate doves and some mainstream voters.
Yet, the political pressure on Mr. Bush to change course doesn't seem that intense.
The White House argues that the president's policies will be vindicated. Americans will see over time that ousting Saddam Hussein's regime in Iraq and planting a democracy at the center of that volatile region will have been good for the United States and the world.
Polls are going to go up and down as periods of calm in Iraq turn to periods of violence, the White House says. But war opposition will never be sustained and overwhelming, and in the long term, the war will be seen as worth fighting.
But there's another argument to be made to explain the anti-war movement's lack of impact. By this reasoning, the president would be in a much tougher spot if the present-day anti-war movement itself were more effective.
Democrats continue struggling to find a cohesive, rallying message on the war. Many of them are constrained by their earlier votes to support the conflict. Many fear that being labeled weak on national security could be more politically damaging than charting out a moderate -- if murky -- message.
Last fall, Pennsylvania Democrat John Murtha, a decorated Vietnam War vet who was known as a lifetime hawk, caught many in Washington by surprise when he emotionally called for the president to withdraw from Iraq.
Murtha's plea was both credible and well articulated. But outside the Washington beltway, he lacked name recognition and star power. To date, he has not shown much popular appeal, and he has been out of the headlines for a while.
Then we come to Cindy Sheehan. When she turned up outside Mr. Bush's ranch in August, some in the anti-war movement believed they had found their Joan of Arc.
The New York Times, in one of its first stories about Sheehan, described her as the modest, mourning mother of a soldier who died in Iraq. She was pictured as an innocent victim of the war who just wanted a meeting with the commander in chief.
The Times ran a picture of her pacing alone on a Texas roadside near the president's ranch. Its report described her standing "beside a small stand of trees and a patch of shade that contained a sleeping bag, some candles, a jar of nuts and a few other supplies." Sheehan told the paper she was tickled by the attention, but pleased that she had "opened up a debate in the country."
Suddenly, Sheehan was all over national television. Overnight, she became a household name. But in the months since her emergence, she has also become a darling of the Left, and she has seemed to seek out attention rather than let it come to her.
She has become a regular on the blog of filmmaker Michael Moore. Just last week, she traveled to Venezuela and met with President Hugo Chavez, the leftist leader who has accused President Bush of trying to have him assassinated.
During the visit, Chavez implored, "Down with the U.S. Empire!" and said that he might join the encampment outside Mr. Bush's ranch next time Sheehan goes.
Sheehan thanked Chavez, praised him as a leader who supports peace and said that the Venezuelan leader told her she should run for U.S. president. She said that made her laugh. (She has, however, been pondering a run for the Senate.)
Following her swing through Venezuela, Sheehan came to Washington last week and attended the president's State of the Union Address. When Sheehan unveiled her T-shirt, displaying the number of fallen soldiers in Iraq, she was deemed a demonstrator by U.S. Capitol Police and removed from the gallery (where any form of demonstration is barred by longstanding rules).
"I wore the shirt to make a statement," Sheehan wrote on www.michaelmoore.com. "The press knew I was going to be there and I thought every once in awhile they would show me and I would have the shirt on. I did not wear it to be disruptive."
Fair enough. But she probably would have drawn the cameras just by being there, so the T-shirt made for a missed opportunity. It's easy to imagine the way the evening might have gone:
"Our work in Iraq is difficult because our enemy is brutal, but that brutality has not stopped the dramatic progress of a new democracy," the president says. Then the cameras turn to Sheehan for her silent reaction.
"The road of victory is the road that will take our troops home," Mr. Bush says. Then the cameras pan again, showing a woman who symbolizes the cost of the Iraq war.
Still, no matter how well or poorly Sheehan and other activists play the media, the anti-war movement will require more than stunts and symbols. It needs a clear and well-articulated message to compete with the president's Sept. 11 mantra and steady emphasis on national security.
That's going to require the emergence of a more visible and credible leader than either the movement or the Democrats have found thus far.