History tells us that vice-presidential debates have no effect on the election as a whole. Not a small effect--none at all.
No matter how well or how poorly a vice-presidential candidate has performed in a debate so far, the polls haven't responded.
After Lloyd Bentsen smacked down Dan Quayle in 1988 with his infamous "You're no Jack Kennedy" line, Quayle spent years living it down. But the polls didn't budge, and he and running mate George H. W. Bush went on to win the election.
In 2008, 70 million people tuned in to watch Sarah Palin's debate debut, and she wowed them with her folksy charm. But even that much exposure did nothing to affect the trajectory of the campaign. Her opponent, Joe Biden, followed his running mate Barack Obama to a historic victory.
But this is no typical election.
Presidential debates aren't supposed to affect the outcome of an election either, but the first presidential debate last week between Obama and Republican challenger Mitt Romney resulted in a 12-point swing in the polls.
Romney's campaign, which had been declared dead-in-the-water just weeks before has now jumped into the lead in most nationwide polls, and there are few signs of his momentum slowing.
So this vice-presidential debate may actually matter. It may determine whether Obama recovers from that terrible night in Denver last week.
Biden will be doing his best to pump up the Democratic base, whose enthusiasm is waning dangerously. Pollsters guess half of Romney's recent gains are due to shifts in enthusiasm in the electorate. Romney is doing well because he is doing well, an odd political tautology usually more applicable to celebrities.
Biden, a known foreign policy buff, will try to hit Republican opponent Paul Ryan hard on his economic and healthcare plans. Democrats were disappointed in the lackluster defense Obama mounted for liberal policies, and Biden will be trying to wave the progressive banner as vigorously as he can.
He's had practice.
It was Biden's speech at the Democratic National Convention in September that had the most viewers, not Bill Clinton's or either of the Obamas'.
For his part, Ryan needs to continue the momentum Romney set into motion last week. Ryan may have some trouble with this, as his public speaking tends to be heavy on figures and statistics. But a Biden gaffe may give him an opportunity to shine.
As for the polls, as long as these two candidates do their jobs and no one falls on their face, it's likely that Romney's momentum will slow a bit and the race will settle into a dead heat until next week, when Obama faces Romney again and gets a chance to redeem himself.
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