WASHINGTON — They spend hours mastering policy, learning to lean on the podium just so, perfecting the best way to label their opponents as liars without whining.
But presidential candidates and their running mates often find that campaign debates turn on unplanned zingers, gaffes or gestures that speak volumes.
Debate victories and losses often are scored based on the overall impressions candidates leave with voters.
In the history books, though, small moments often end up telling the broader story.
Think of 73-year-old Ronald Reagan's pledge not to exploit his opponent's youth. Or John McCain's dismissive reference to Barack Obama as simply "that one."
A look back at some of the best and worst exchanges from presidential debates.
REAGAN: "I am not going to exploit, for political purposes, my opponent's youth and inexperience."
When Reagan won the White House in 1980, he was 69 and the oldest man elected to the office. During his successful 1984 re-election campaign, he faced questions about his age in a contest against 56-year-old Walter Mondale, the former vice president.
One of the debate's moderators, Baltimore Sun diplomatic correspondent Henry Trewhitt, cast Reagan's age as "an issue that I think has been lurking out there for two or three weeks" and a matter of national security.
"You already are the oldest president in history and some of your staff say you were tired after your most recent encounter with Mr. Mondale," Trewhitt said. "I recall yet that President Kennedy had to go for days on end with very little sleep during the Cuban missile crisis. Is there any doubt in your mind that you would be able to function in such circumstances?"
Reagan was ready with his retort.
"I want you to know that, also, I will not make age an issue of this campaign. I am not going to exploit, for political purposes, my opponent's youth and inexperience," he said to laughter. "If I still have time, I might add, Mr. Trewhitt, I might add that it was Seneca or it was Cicero — I don't know which — that said, 'If it was not for the elders correcting the mistakes of the young, there would be no state.'"
LLOYD BENTSEN: "Senator, you are no Jack Kennedy."
George H.W. Bush's pick for vice president, fresh-faced Sen. Dan Quayle of Indiana, got multiple questions about his preparedness to assume the office during a 1988 debate.
"I want to take you back to the question that I asked you earlier about what would happen if you were to take over in an emergency, and what you would do first and why," moderator Brit Hume told Quayle. "You said you would say a prayer, and you said something about a meeting. What would you do next?"
"I don't believe that it's proper for me to get into the specifics of a hypothetical situation like that," Quayle replied.
Moderator Tom Brokaw later pressed Quayle: "If you cite the experience that you had in Congress, surely you must have some plan in mind about what you would do if it fell to you to become president of the United States, as it has to so many vice presidents just in the last 25 years or so."
"Three times that I've had this question," a frustrated Quayle said before teeing up a rehearsed answer.
"I have far more experience than many others that sought the office of vice president of this country. I have as much experience in the Congress as Jack Kennedy did when he sought the presidency."
Quayle's Democratic rival, Texas Sen. Lloyd Bentsen, pounced: "Senator, I served with Jack Kennedy. I knew Jack Kennedy. Jack Kennedy was a friend of mine. Senator, you are no Jack Kennedy."
"That was really uncalled for, senator," replied Quayle.
"You are the one that was making the comparison, senator," Bentsen shot back. "And I'm one who knew him well."
Bentsen, running mate to Michael Dukakis, scored the win in that debate, but the Democrats nonetheless lost the election to Bush.
McCAIN: "That one."
McCain's campaign was on the ropes when he faced Obama for a second debate in 2008. He had suspended his campaign the previous month to rush back to Washington and address the Wall Street crisis. But he came off as unprepared to handle the economy, suddenly voters' top concern. Polls showed McCain was slipping behind. And his on-the-ground operations were badly lagging Obama's.
So when Brokaw asked McCain about government-funded energy innovation, McCain saw his chance to take a detour and paint Obama as a spend-happy Washington insider.
"By the way, my friends, I know you grow a little weary with this back-and-forth. It was an energy bill on the floor of the Senate loaded down with goodies, billions for the oil companies, and it was sponsored by (President George W. Bush) and (Vice President Dick) Cheney. You know who voted for it? You might never know," McCain said, gesturing to his rival.
And that was a turnoff for many voters.
GERALD FORD: "There is no Soviet domination of eastern Europe."
During his 1976 debate against Democrat Jimmy Carter, President Ford seemed to ignore the political reach of the Soviet Union, so much so that the debate moderator did a double-take. Republicans and Democrats alike recognized that Eastern Europe was largely under Soviet influence but the United States strongly opposed any efforts that would take the land and put it within their borders.
"There is no Soviet domination of Eastern Europe and there never will be under a Ford administration," Ford said.
"I'm sorry, did I understand you to say, sir, that the Russians are not using Eastern Europe as their own sphere of influence occupying most of the countries there, and making sure with their troops that it is a Communist zone whereas on our side of the line the Italians and the French are still flirting with the possibilities?" asked Max Frankel of The New York Times.
Carter, who went on to defeat Ford, challenged the president's views on Eastern Europe.
"I would like to see Mr. Ford convince the Polish-Americans and the Czech-Americans and the Hungarian-Americans in this country, that those countries do not live under the domination and supervision of the Soviet Union behind the Iron Curtain," Carter said.
BODY LANGUAGE: Watches, sighs and eye rolls
Sometimes, the most memorable moments are wordless.
Take the first President Bush in 1992. The incumbent looked down at his watch as a voter asked a question about the struggling economy during a town hall-style debate, suggesting he was bored with the whole thing.
Bush lost his re-election bid to Clinton.
Vice President Al Gore sent an over-the-top message of impatience during his 2000 debate with then-Texas Gov. George W. Bush with a series of loud, overwrought sighs.
Gore also overdid his incredulity toward some of Bush's claims during their first debate.
The vice president rolled his eyes, a move that his staff — and America — instantly recognized as problematic. Before the next debate, Gore's aides made him watch a "Saturday Night Live" skit mocking his performance.
Course correction was not enough. Gore lost to Bush.