In presidential politics, everybody's searching for "the moment." The campaigns don't know when or how it will come, but they watch for something — awkward words, an embarrassing image — that can break through and become the defining symbol of the other guy's flaws.
Now all eyes will be on the three presidential debates, especially the all-important first one Wednesday night, a perfect incubator for such a moment.
The unpredictable nature of the debates is part of what attracts tens of millions of live-TV viewers. A big blunder or "gotcha" quip is sure to be remembered. And President Barack Obama and challenger Mitt Romney will be fielding questions for 4½ hours in all, a long time to stay on message.
"If we have any moment in terms of seeing the true, the real, the unscripted candidate, it's likely to happen on the debate stage," said Mitchell McKinney, a University of Missouri associate professor who studies the presidential match-ups.
It's the all-too-human slips that live on. Think of Al Gore sighing loudly and often, a bored-looking George H.W. Bush checking his watch, or Richard Nixon appearing clammy and tired next to a tanned and rested John F. Kennedy in the first televised debate in 1960.
"Whether it's sighing or looking at your watch, people will remember that. And they'll have a lot of help because the press is going to replay it and replay it and it's going to show up on 'Saturday Night Live,'" said Tad Devine, who was a senior adviser to Gore's campaign. "It becomes part of their daily conversation, and it takes hold."
The killer moments are the ones that seem to verify what voters already suspect, Devine said, usually after weeks or months of priming by the opposing camp.
Romney and Obama know the risk firsthand.
During the GOP primary debates, Romney played into worries that his wealth distances him from ordinary people by offering to back his words with a friendly wager — a $10,000 bet.
In the 2008 Democratic primaries, Obama reinforced criticism that he's arrogant and aloof. As Hillary Rodham Clinton good-naturedly tried to deflect a question about her likability, Obama cut in. "You're likable enough, Hillary," he said dismissively. He didn't even look up.
A defining moment can sprout anywhere, of course.
For John McCain, it was a Florida campaign rally on Sept. 15, 2008. As Lehman Brothers filed for bankruptcy, the nation's financial system teetered on the brink and stocks tanked, McCain insisted that "the fundamentals of our economy are strong." The Obama campaign jumped on that phrase to portray McCain as oblivious — and it stuck.
A question from a college student about funding for the wars in Iraq and Afghanistan tripped up John Kerry in 2004. "I actually did vote for the $87 billion before I voted against it," Kerry declared. President George W. Bush's campaign had what it needed: the perfect shorthand for Kerry as a flip-flopper.
This year's campaigns already have spent months looking for that one devastating blow. So far the top contender is Romney's secretly recorded dismissal of "47 percent of Americans" who don't pay taxes and depend on government aid. And then there's Obama's "you didn't build that" remark about business owners.
This year's first presidential debate, focused on the economy and the role of government, offers each man a chance to explain away such mistakes — and the risk of cementing a new one into voters' minds.
Brett O'Donnell is the debate coach credited with teaching Romney how to deflate Newt Gingrich during the Florida primaries. He says that on Wednesday the GOP nominee needs to confront his video-recorded setback head-on and turn it against Obama.
"He has to turn the 47 percent and make sure people understand the reasoning behind that argument, how that's the result of the president's policies," he said. Romney can emphasize the increasing number of people relying on food stamps, unemployment checks and other aid, suggests O'Donnell, who no longer works for the campaign
The big moments don't have to be bad, after all.
It was Republican strategist Lee Atwater who promoted the idea of a moment that defines a candidate's character. He saw a positive one when Vice President George H.W. Bush stood up to CBS anchorman Dan Rather in a live interview about the Iran-Contra scandal. The aggressive performance quashed criticism that Bush was wimpy.
In his 1980 challenge to President Jimmy Carter, Ronald Reagan pulled off two famous debate lines that helped win a landslide. "There you go again," he quipped when Carter accused him of planning Medicare cuts. And he asked America a question that still echoes today: "Are you better off now than you were four years ago?"
Reagan's shaky performance in the first debate of 1984 raised questions about the 73-year-old president's fitness for another term. Next round, when asked about the age issue, Reagan silenced doubters by declaring with a twinkle that he would not "exploit for political purposes my opponent's youth and inexperience."
At that moment, "the campaign was over," the Democratic nominee, Walter Mondale, later told PBS journalist Jim Lehrer.
Debates rarely yield such total triumphs, however. And they're seldom an election's single deciding factor, scholars such as McKinney say.
President Gerald Ford's insistence in 1976 that "there is no Soviet domination of Eastern Europe" undermined confidence in his ability as a Cold War leader. Texas Gov. Rick Perry wiped out his chance at this year's Republican nomination by forgetting the third of three federal agencies he wanted to eliminate. All he could say was, "Oops."
Although it doesn't happen often, McKinney said, "There is a chance to blow it."