(CBS NEWS) -- When Rep. Paul Ryan and Vice President Joe Biden square off in Thursday night's vice presidential debate in Danville, Ky., sparks will fly, consultants will fret, and snap polls will deliver snap judgment. Twitter might actually explode. And come November 6, it probably won't make a difference.
Despite screaming headlines predicting a "high-stakes" encounter, the VP candidates can take solace in this, at least: their performance, barring a spectacular implosion or gaffe, is unlikely to affect the outcome of the 2012 election.
The potential impact on Biden and Ryan, however, could be profound.
Alan Schroeder, a professor of journalism at Northeastern University and author of "Presidential Debates: 50 Years of High-Risk TV", argues that the VP debate "has more to do with the legacy for the debater than affecting the outcome of peoples' voting decisions."
Indeed, a look at past vice presidential debates reveals an event that scarcely affects the election at hand but can render a powerful, durable verdict on the sparring understudies.
The stakes may be higher for the 42-year-old Ryan, a young up-and-comer commonly heralded as the vanguard of a new generation of Republican leaders. A strong performance from Ryan would solidify his standing as the GOP's heir-apparent; a tepid performance could tarnish his credibility as a big thinker and derail his national ambition.
But Biden, too, will be watched with an eye to 2016. Despite abundant skepticism among the commentariat, Biden confidantes insist that the loquacious 69-year-old must be seen as a viable presidential candidate four years hence. An aggressive, punchy performance could elevate his stature within the party, quelling doubts about the vice president's age and competence. A feckless night could erase whatever remains of Biden's 2016 currency.
John Sides, a professor of political science at The George Washington University, explains that the debate can be an audition for bigger things: "If a vice-presidential candidate has grander ambitions, their performance as a candidate is one factor that could affect their prospects." Though he argues that "party leaders will look at a VP debate as one data point" in measuring the presidential caliber of rising stars, he cautions, "It's not the only one, or even the most important one."
In the first vice presidential debate of the television era, a 1976 face-off pitting then-Sen. Bob Dole, R-Kan., against then-Sen. Walter Mondale, D-Minn., Dole delivered the night's most memorable attack, scorching Mondale and the Democrats as warmongers: "I figured it up the other day, if we added up the killed and wounded in Democrat wars in this century, it'd be about 1.6 million Americans - enough to fill the city of Detroit."
Dole had developed a reputation for acidity, as Mondale reminded the audience: "Senator Dole has richly earned his reputation as a 'hatchet man' tonight." The "hatchet man" label endured for decades.
Before too long, however, the attack dog would be leading the pack - first as Senate Republican leader during the 1980s and '90s and then as a presidential nominee in 1996. Sharp elbows rarely win friends, but they can earn respect.
In 1984, the Democrats' vice presidential nominee was New York Rep. Geraldine Ferraro, the first woman on a major party's national ticket. Her opponent, Vice President George H.W. Bush, a World War II veteran, former ambassador and C.I.A. chief, enjoyed tremendous stature but proved unable to keep his supercilious posture offstage, deigning at one point to "Help you with the difference, Ms. Ferraro, between Iran and the embassy in Lebanon."
Ferraro fired back: "I almost resent, Vice President Bush, your patronizing attitude that you have to teach me about foreign policy."
At the end of the debate, Bush's self-congratulation was caught on an open mic (the Republican thought he "kicked a little ass"), but Ferraro's defiant riposte emerged as the evening's most memorable line.
Schroeder argues that Bush, chagrined by his encounter with Ferraro, became gun-shy: "He never liked to do debates, and he got off to a bad start with Ferraro. Four years later, people remembered, and he remembered. And it became part of his reticence about having to take part in presidential debates."
The 1988 vice presidential debate provided perhaps the clearest illustration of the event's short-term impotence and long-term resonance. The GOP vice presidential nominee, 41-year-old Indiana Sen. Dan Quayle, faced doubts about his experience and youth.
Quayle needed to close the stature gap, but when he dared equate his congressional career with that of the late President John F. Kennedy, his opponent, Sen. Lloyd Bentsen, D-Tex., seized the moment. Eyes twinkling, Bentsen unleashed a crushing broadside: "Senator, I served with Jack Kennedy. I knew Jack Kennedy. Jack Kennedy was a friend of mine. Senator, you're no Jack Kennedy." The audience erupted as Quayle, stone-faced but visibly peeved, refused to even look at his opponent.
Bentsen's putdown did not change the trajectory of the election -- Bush and Quayle cruised to a big victory -- but Schroeder suggests that it may have buried Quayle's national ambition: "Dan Quayle had probably the worst moment of any VP debater ever. That comment really sunk his presidential chances."
Bentsen permanently handicapped Quayle's reputation by providing an easy frame for future gaffes -- he had slammed Quayle as a dilettante, and the label eventually stuck. By the time Quayle was caught on tape misspelling "potato" (no "e") in 1992, nobody was surprised. His future flirtations with national office went precisely nowhere.
The 2000 vice presidential debate, pitting then-Democratic Sen. Joseph Lieberman against former Defense Secretary Dick Cheney, was gripped by an abiding propriety -- veteran debate moderator Jim Lehrer called it "the most civilized debate anyone could remember."
Lieberman's refusal to aggressively prosecute Cheney seemed wise at the time -- advisers had counseled him to remain civil. But in retrospect, Schroeder, argues, "Lieberman got rolled. He didn't even realize that it was happening. Cheney gave one of the greatest VP debate performances of all time in 2000."
"There was a lot of consternation about why Cheney was on the ticket -- perhaps Bush made an error in choosing him. Lieberman, going in, was supposed to be a charismatic guy, ready with a quip. So the debate showed a new side of each of them."
In 2004, with the help of Vice President Cheney, the Democratic VP nominee, then-Sen. John Edwards, emerged from his debate looking vacuous and perhaps a bit slimy. Cheney criticized Edwards' spotty attendance record in the Senate, exclaiming, "The first time I met you was when you walked on this stage tonight." Subsequent reports revealed Cheney's claim to be untrue, but it was not the last time Edwards would be tagged as callow.
When Edwards later referenced Cheney's lesbian daughter Mary in response to a question about same-sex marriage, some analysts were discomfited. Although Dick Cheney demurred onstage, refusing to respond to the comment, Mary Cheney was not so charitable, calling Edwards "complete and total slime" in an interview.
Despite all the sturm and drang, the vice presidential debate frequently amounts to very little. Dole, Ferraro, and Bentsen each delivered aggressive performances that failed to advance their immediate electoral prospects.
But for the wannabe-veeps onstage, the night can help make the difference between a bright political future and a twilit turn on the speakers' circuit. Quayle, Lieberman, and Edwards were all diminished in some way by their big night.
In 1980, four years after the first televised vice presidential debate, the sequel was grounded before takeoff. Republican VP nominee George H.W. Bush justified his refusal to debate Vice President Mondale by damning the significance of the proposed event: "We're the Toledo Mud Hens," explained Bush, colorfully relegating the aborted showdown to the minor league.
Still, it may be worth remembering that some of the Toledo Mud Hens eventually went pro, proving that even the farm team can make major league waves.