More often than not the debates have been standoffs, not turning points. Debates have done more to reinforce trends and reputations than to change them.
But when two candidates go head to head live, you just never know.
And when Republican John McCain and Democrat Barack Obama meet Friday at the University of Mississippi for the first of their three debates, the pressure on them will be as intense as it gets in politics. The audiences are vast, up to 70 million people in prior campaigns.
Nixon and JFK inaugurated the age of TV presidential debates in 1960, also on Sept. 26, with the first of four debates between them and the only one that made a significant difference.
Both men were well-spoken. But Nixon looked hollow-eyed and dark-bearded. His gray suit blended into the gray studio background. Kennedy was tanned and relaxed, and wore a blue suit that provided a good contrast to the black and white screen. That's what ended up mattering.
Nixon figured the debates would become a fixture in presidential campaigns. They did not, for 16 years.
In 1964, President Lyndon B. Johnson wasn't interested in sharing a stage with far-back Republican Barry Goldwater. In 1968, Nixon was the Republican nominee and avoided debating Democrat Hubert H. Humphrey. Four years later, Nixon ducked debates again as he cruised to re-election against George McGovern.
Then, in 1976, President Ford found himself in need of a forum to make headway against Democrat Jimmy Carter, and the presidential debate series was revived. In their first debate, the sound system failed in Philadelphia, leaving the nominees standing there for 27 minutes, silently sweating in the TV lights.
In the second set, Ford made the great blunder of the great debates by claiming "there is no Soviet domination of eastern Europe." That was wrong, but Ford wouldn't correct himself. He'd been gaining on Carter, but no more. The mistake reinforced Ford's reputation as a man of missteps, the guy who tripped on airplane steps, who couldn't walk and chew gum. It wasn't fair, but it stuck.
In 1980, President Carter was the debater on the defensive, up against the polished Republican Ronald Reagan, who countered each criticism with a weary "There you go again." Style, not substance, but it worked in their one debate. Reagan had debated earlier against independent John B. Anderson; Carter wouldn't come that night. So they spent most of an hour complaining that he wasn't there.
Nor did Carter help himself by saying in the Reagan debate that he'd discussed nuclear weapons control with his daughter Amy, then 12. That led to Republican ridicule.
In 1984, Reagan rambled through a halting, confused performance in his first debate against Democrat Walter F. Mondale. He was 73 and his age was becoming a concern.
The president neutralized that with a quip in the next debate, when he was asked if he had the strength for the job at his age. "I will not make age an issue of this campaign," he said. "I am not going to exploit for political purposes my opponent's youth and inexperience." Even Mondale had to laugh.
Time will tell whether McCain, who at 72 would be the oldest man entering the White House, will have such a zinger. It would be harder to pull off, given the youth and inexperience of his own running mate, 44-year-old Sarah Palin.
The memorable moment of the 1988 debates was an ambush question aimed at Democrat Michael Dukakis. A TV panelist asked Dukakis whether his opposition to capital punishment would stand if his wife were raped and murdered. Dukakis answered dispassionately and stood by his position. It fit the picture of Dukakis as a cold technocrat. All George H.W. Bush had to do was watch.
In 1992, nothing Bush said damaged him more than when the cameras caught him checking his watch in a Richmond, Va., debate with Bill Clinton and Ross Perot. He looked tired of the whole business. In that campaign, it often seemed he was. He'd never cared for debates. "Now was I glad when the damn thing was over?" he asked later. "Yeah."
The 1996 debates between President Clinton and Bob Dole offered little to distinguish them. But the expectations game was by then already in full swing. So when Dole performed better than expected, by some accounts it was a draw.
So, too, in 2000, when flub-prone George W. Bush went up against Gore, a practiced debater and policy wonk. Bush held his own. Gore shrugged and sighed as Bush spoke, then backed off into a sedate, almost deferential style, then approached Bush aggressively. Bad show. Style points count.
Four years later, President Bush's verbal blunders, glares and smirks played poorly against Democrat John Kerry. With Kerry gaining on him, Bush got his edge back in their final debate by claiming a Democratic victory would weaken national security.
This year, the Oct. 2 debate between vice presidential nominees Palin and Joe Biden is as eagerly anticipated as the matchups at the top of the ticket.
That's fitting - some of the punchiest lines have been in vice presidential debates.
In 1976, Dole famously said that all wars of the 20th century were "Democrat wars." Dan Quayle stumbled into a comparison of his congressional experience with John Kennedy's - they were about even - which teed it up for Democrat Lloyd Bentsen. "Senator," Bentsen said, "you're no Jack Kennedy."
Ross Perot's running mate, James B. Stockdale, took his turn against Quayle and Gore in 1992.
"Who am I?" he asked. "Why am I here?" He had to wonder himself. He'd only been told about the debate a week before.