By Max Deveson
BBC News, Washington
They have provided some of the most memorable moments in recent US political history.
Al Gore's eye-rolling.
Lloyd Bentsen's reminder to Dan Quayle that he "knew Jack Kennedy".
Ronald Reagan's "There you go again" directed at Jimmy Carter.
But do the televised presidential debates have any impact on the way people actually vote?
"Once you get out of the convention period," writes polling analyst Nate Silver, "voter preferences tend to have become a lot more stubborn, and even terrific or terrible debate performances don't tend to alter them all that much."
Tom Holbrook, professor of political science at the University of Wisconsin-Milwaukee, has conducted a study of the effects on public opinion of all 13 presidential debates since 1988.
He found that - for the most part - individual debates moved the opinion polls very little.
"Across all 13 presidential debates, the average absolute change in candidate support was one percentage point," he writes.
He did find that public opinion shifted more substantially over the course of the debates, however.
Ronald Reagan's poll numbers shot up after his debate with Jimmy Carter
So, for example, by the end of the debate period in 1988, George H W Bush had increased his lead by 2.42 percentage points, Al Gore's popularity slumped by 3.52 points in 2000, and George W Bush's poll lead dropped by almost 2 points in 2004.
In a close election - as in 2000 - the debates can make the difference between winning and losing.
In 1980, the debate between Jimmy Carter and Ronald Reagan seemed to make all the difference between a close race and a landslide.
Before the debate, the candidates were neck-and-neck in the polls.
If anything, Mr Carter, the White House incumbent, was thought to have a slight advantage.
Throughout the summer, the two campaigns had been wrangling over the format of the debates.
Mr Carter had refused to take part if independent John Anderson also took part.
Mr Reagan said he would participate only if Mr Anderson were allowed to join in.
Eventually, with one week to go before the election, Mr Reagan gave in and agreed to debate Mr Carter without Mr Anderson.
Observers agreed afterwards that Mr Reagan had had the better of the debate.
He was optimistic where Mr Carter was gloomy, quipping "there you go again" when Mr Carter attempted to bring up his voting record on Medicare and social security.
And with one killer question - "Are you better off now than you were four years ago?" - Mr Reagan struck a chord with watching voters.
Internal polls conducted by the Carter campaign suggested the president's popularity had dropped five points after the debate.
And because the debate took place so close to election day, Mr Carter had little opportunity to get back into the race.
These days, candidates allow a little more time between the last debate and the election.
This year the last debate will be on 15 October, almost three weeks before polling day, so the candidates will have plenty of time - and news cycles - to repair any damage caused by a poor performance.
Ahead of the first debate, which is set for Friday, opinion polls suggest that Mr Obama is - narrowly - in the lead.
But as Professor Holbrook says: "Even relatively small shifts in the same direction over the three debates could make this relatively tight race even tighter (if the shifts favour McCain), or could blow it open (if the shifts favour Obama)."
In other words, a good performance in the debates may not - on its own - be an election-winner for Mr McCain, but it could even the race; a good showing from Mr Obama could put him - like Ronald Reagan before him - on course for the White House.