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.

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VP debate impact may be more personal than electoral