(CBS News) Recent polls show President Barack Obama with a small but increasing lead - so the conventional wisdom in Washington is that challenger Mitt Romney needs this first debate to be a game changer. What are the chances of that happening?
After the first-ever televised debate in 1960, regular general election face-offs began in 1976. They've produced some memorable gaffes, but few instances which really changed the outcome of the election.
In 1980, President Jimmy Carter led Gov. Ronald Reagan in the polls, but one line from Reagan in their only debate put Carter on the defensive - and became a mantra for future challengers to an incumbent: "Are you better off than you were four years ago?"
In 1984, Reagan, then the incumbent, did it again. After seeming to lose his way in the first debate with Walter Mondale, the 73-year-old Reagan defused the issue with one, well-timed quip.
Reagan said, "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."
But memorable one-liners like those are few and far between.
Historian Doris Kearns Goodwin says it's not what the candidate says that usually matters most. "Less important than what they say is how they appear," she said. "Do they treat their rival with respect? Do they connect with the audience? Do they respond at the moment when they have to react? How do they look? How do they seem?"
During the first-ever televised debate, Richard Nixon chose to wear no makeup. With a five o'clock shadow, he looked sweaty and uncomfortable compared to the tan, relaxed John F. Kennedy.
In 2000, voters watching the debate heard the impatient sighs of Vice President Al Gore captured clearly by the microphones while George W. Bush was talking.
Both moments played into a larger narrative of the campaigns by reinforcing what viewers already thought about the candidates.
Kearns Goodwin said, "When Gore sighed endlessly and moaned during the debate and we saw that on television it just emphasized the idea that he was arrogant and condescending, something people were already concerned about. When Nixon was sweating, there was some sense that he was already shifty and there was an anxiety in his soul as well as his body."
In a later debate between Gore and Bush, Gore appeared to invade the personal space of Bush - a move which made him look awkward compared to the relaxed Bush.
And in 1992 - George H.W. bush was caught twice by cameras glancing at his watch during a town hall debate with voters, which reinforced the suggestion that he was disengaged and uninterested.
Kearns Goodwin said, "When something's close as it is now, a small shift may create a sense of forward movement in one direction or another, so that's why these debates become great moments."
Look for the unscripted moments: a startled reaction, a moment of apparent condescension, a verbal misstep - any of which could give one candidate an advantage going forward.