It’s a truism that no one votes for president based on the vice presidential candidates, so it should follow that a debate between VP nominees is about as meaningless as a preseason football game.

But that’s not quite true. Yes, few (if any) voters actually stare at their ballots weighing the pros and cons of each vice presidential hopeful. But that kind of analysis is too literal. The performance of VP nominees affects the evolution of swing voters’ attitudes toward the presidential nominees during the campaign – and, by far, the debate marks the highest visibility moment for any VP candidate. Sometimes, this still doesn’t matter; when there is a mismatch at the top of the ticket, even an awful VP candidate (like Dan Quayle in 1988) can’t reverse the dominant trend. But in a close election, VP nominees can offer reassurance to – or raise new doubts among – voters who are still making up their minds.

A look back at some moments that defined the previous VP debates – and whether they made any difference on Election Day:

1976:The first ever VP debate, between Bob Dole and Walter Mondale, may have had the biggest impact on a race’s outcome. Dole had been tapped by Gerald Ford to appeal to conservative Republicans (who had lined up with Ronald Reagan in the primaries) and to shore up heartland states for the G.O.P. ticket. But he showed none of the good humor for which he later became known on the campaign trail, instead earning a reputation for nastiness and mean-spiritedness (like when he called Jimmy Carter “southern-fried McGovern” during one speech).

The Ford-Dole ticket was on the move when the VP debate was held, having cut a 33-point deficit into single digits. But Dole created a severe headache for the ticket and slowed its momentum when he answered a question about Watergate by declaring: “I figured out the other day: If you added up the killed and wounded in Democrat wars this century, it would be about 1.6 million Americans – enough to fill the city of Detroit.”

Mondale simply responded: ‘I think Senator Dole richly earned his reputation as a hatchet man,” a characterization that would haunt the Kansan for years to come. Whether Dole’s bitter performance was the difference maker on Election Day is an open question. Ford ended up catching Jimmy Carter over the final weekend, only to watch Carter surge ahead in the final three days and win by two points. In defeat, Ford’s aides pointed the finger at Dole – but it’s just as likely that the last-minute prospect of a Ford victory (he’d been given up for dead for most of the campaign) gave voters pause.

1984:The year that VP debates became a mainstay, the 1984 showdown between Geraldine Ferraro and George H. W. Bush showdown was widely anticipated because of Ferraro’s status as the first woman ever nominated for national office by a major party.

The debate was held just days after Ronald Reagan and Walter Mondale’s first forum, which featured a doddering performance by the 73-year-old Reagan that unnerved millions of Americans and (however briefly) added some uncertainty to the campaign. Reagan would recover his footing in the second debate and quickly restore his lopsided (around 20 points) advantage over Walter Mondale, so it can’t really be said that the Ferraro-Bush debate mattered on Election Day – the top-of-the-ticket contest was too uncompetitive.

On points, Bush and Ferraro probably debated to a draw. But the most riveting moment came when Bush, the incumbent vice president, seemed to condescend to Ferraro, offering to help her understand Middle East Policy. “I almost resent, Vice President Bush, your patronizing attitude that you have to teach me about foreign policy,” Ferraro replied to applause.

It was one of several gender-related gaffes that week from Bush and his campaign. The day after the debate, he boasted to a group of longshoremen in New Jersey that he had “kicked a little ass last night,” while his wife, Barbara, had before the debate called Ferraro “a four-million dollar – well, I can’t say it, but it rhymes with ‘rich.’” (Bush later claimed that the word she’d been thinking of was ‘witch.’) Also just before the debate, Bush’s press secretary, Pete Teeley, told the Wall Street Journal that Ferraro was “too bitchy.” In a closer campaign – and in more recent years – this all might have mattered, but not so much in 1984.

1988:Easily the most famous VP debate moment ever, and quite possibly the most memorable moment in any debate of the television age – Lloyd Bentsen’s immortal response to Dan Quayle after Quayle, for the third time of the night, compared his own experience to that of John F. Kennedy when Kennedy was elected president:

The ’88 debate stands as the textbook example of the limits on a VP candidate’s Election Day influence. That one moment cemented Quayle’s reputation (already well earned) as a lightweight, just as it transformed Bentsen into a wildly popular political celebrity (he very well could have had the 1992 Democratic nomination, but declined to seek it). But Quayle’s ticket still won 40 states and 426 electoral votes. The mismatch at the top of the ticket between George H. W. Bush and Michael Dukakis was too much for Bentsen or Quayle to disrupt. In a close election, though, Quayle’s performance could have been devastating to the G.O.P. – just as Bentsen’s probably would a have been decisive for the Democrats.

1992:Poor James Stockdale. An authentic American hero, the highly decorated Naval officer (who spent seven years in a North Vietnamese prison camp) never wanted to get into politics, but agreed to serve in the spring and early summer of 1992 as a stand-in running mate for Ross Perot. The idea was that Stockdale would lend his name to Perot’s petitioning effort (some states required that a VP candidate be named for petitions to be accepted) and then drop out once Perot found a permanent running mate. But Perot abruptly dropped out of the race in mid-July (although he made sure to leave his name on all ballots), and when he re-emerged at the start of October, he hadn’t found a new running mate. So the gig – and a slot in that fall’s VP debate – went to Stockdale.

The rest is history. Stockdale delivered the most memorable opening line ever (“Who am I? Why am I hear”), had to ask for a question to be repeated because he’d turned his hearing aid off, paced back and forth behind his podium during questions (a result of his war injuries – not that viewers at home knew this), and became an enduring pop culture punch line, as this clip from a Saturday Night Live episode from the fall of 1992 illustrates:

Of course, there were two other VP candidates on stage with Stockdale – Al Gore and Dan Quayle. Both won solid reviews – a surprise in Quayle’s case, given his hideous performance in 1988.

1996:

Probably the most forgettable VP debate on record. Republican true believers were deflated seconds into the debate when Jack Kemp made a show of announcing that he wouldn’t be playing the traditional “attack-dog” role and that he didn’t see it as his job to raise questions about Bill Clinton’s ethics and personal conduct as president.

“In my opinion, it is beneath Bob Dole to go after anyone personally,” Kemp said.

Of course, Dole had taken this same posture in his first debate with Bill Clinton, an equally dull affair a few nights earlier in Hartford. But many Republicans, and some of Dole’s own advisers, believed that Kemp needed to play hardball, given that the ticket was desperately behind and in need of a jolt. Instead, he and Al Gore traded a few lame jokes and put the audience to sleep.

2000:

Another drab affair, the Dick Cheney-Joe Lieberman debate was more consequential than the ’96 showdown. Both candidates showed dull styles, but Cheney was also surprisingly humorous, flashing a quick wit on two occasions – both of which were repeated over and over on television and radio after the debate. (When Lieberman noted that Cheney, who made a fortune as an oil executive in the 1990s, was better off than he’d been eight years earlier, Cheney quipped, “I can tell you, Joe, the government had absolutely nothing to do with it”; and when Lieberman said that his wife was in the audience thinking, “Gee, I wish he would go out into the private sector,” Cheney replied, “Well, I’m going to try to help you do that, Joe.”)

Cheney’s performance came as he and George W. Bush were enjoying some momentum, the result of widespread criticism of Al Gore’s performance in the first presidential debate, and his humor came as a surprise to most viewers. On the whole, Cheney’s showing was a plus for Bush, maintaining the ticket’s momentum, while Lieberman’s reluctance to attack frustrated Democrats.

2004:

For the second time in as many debates, Dick Cheney won a debate that no one thought he would, this time getting the better of John Edwards, whose smooth communication skills had been a major reason for his selection as John Kerry’s running mate.

But Edwards, like Lieberman in 2000, seemed reluctant to go on the attack and didn’t live up to his oratorical reputation. But he did see fit to bring up Cheney’s lesbian daughter Mary -- by way of complimenting Dick and Lynne Cheney for standing by her -- which created a distracting (for the Democrats) stir after the debate. (Kerry didn’t seem to notice and went on to invoke Mary Cheney in the next presidential debate, which in turn allowed the Cheney family to play the victim card – again distracting the Democrats from their message.)

Bob Shrum, who ran Kerry’s campaign, has since speculated that Edwards had his eye partly on 2008 when he squared off with Cheney and didn’t want to sully his own image by appearing too negative. An example of Edwards’ passivity:

The Oct. 2 debate between Joe Biden and Sarah Palin is easily the most anticipated since 1984 for two reasons. One is that the presidential race is close and the public is unusually engaged. But the bigger reason is the unpredictability of both candidates.

Palin has humiliated herself in a series of interviews with Katie Couric and Charlie Gibson, confirming suspicions that she lacks even a basic grounding in some key policy issues. That the McCain campaign has (besides the aforementioned interviews) sealed her off from press scrutiny has further raised doubts about her ability to think on her feet and to answer simple questions without a script. This would seem to be the recipe for a repeat of Quayle's horrific deer-in-the-headlights performance from 20 years ago. Then again, maybe the bar has been lowered so much that even an average performance from Palin will be greeted by voters as a pleasant surprise.

Biden, for his part, enjoys a reputation for loquaciousness – and for putting his foot in his mouth on sensitive subjects like race. Many believe he'll snatch defeat from the jaws of debate victory on Thursday by condescending to Palin – as George H. W. Bush once did to Geraldine Ferraro – or by letting loose some unfortunate remark that dominates the post-debate coverage. Then again, Biden's debating history suggests his critics might be underestimating his instincts and his self-control – and setting the bar too low for him.