Tonight could be the rare vice presidential debate that actually matters.
Joe Biden and Paul Ryan sit side by side in Kentucky for 90 minutes before a national audience, at a critical point in the campaign where Barack Obama needs a boost. After the president's listless performance against Mitt Romney last week, national and battleground state polls show the former Massachusetts governor pulling closer or ahead of the incumbent who just two weeks ago looked poised to stroll to a second term.
It's too early to know how significant or long-lasting Romney's post-debate bounce really is, but the Obama-Biden campaign can't afford to wait until the president's second debate next week to stem the momentum of Romney-Ryan.
Here are five things to watch:
1Biden's belligerence. For good reason, Republicans love to laugh at Biden's knack for gaffes on the stump and in interviews, but the former U.S. senator from Delaware happens to be an excellent debater. He may be a bit rusty, but throughout 2007 he consistently stood out in the crowded Democratic primary debates featuring Obama and Hillary Clinton.
Four years ago, Biden, 69, went out of his way to be polite and respectful to Sarah Palin in their vice presidential debate, but this time there is little incentive for Biden to worry about coming off condescending.
"The vice president's going to come at me like a cannonball," Ryan predicted this week. Ryan's probably right.
The question is whether Biden overdoes it or veers off script in the process.
2 A heartbeat away. Ryan has spent 14 years in Congress, oversees the budget and is widely seen as an intellectual leader in the GOP with an ability to explain complex budget issues.
But he is just 42 years old, and far more accustomed to giving scripted speeches and committee testimony than standing before a national audience on TV. For all the rehearsals and carefully plotted answers, the debate offers at least an opportunity to see Ryan pushed into some spontaneity.
People vote for presidents, not vice presidents. But they want to see a running mate that is equipped to step into the top job. Dick Cheney came off as far more prepared than John Edwards in 2004, and Dan Quayle never really recovered from his 1988 debate against Lloyd Bentsen.
Ryan is no Quayle. But in this high stakes forum, viewers should have a better sense of how much he deserves his reputation for substance and straight talk.
3 $716 billion. Here's something both Ryan and Biden agree on: The Affordable Care Act cut $716 billion in Medicare spending growth — just as Ryan's own House budget proposal did. Ryan and Romney have nonetheless attacked that cut, even though it does not trim benefits.
Part of what raises the stakes of tonight 's debate is that this is an unusual campaign where the running mate's agenda is a central part of the campaign. Ryan was a strong advocate of letting taxpayers put their Social Security into private investment accounts, and his budget plan could give younger voters the option of taking a health insurance voucher instead of traditional Medicare coverage.
Ryan and Romney have yet to explain how their tax and budget plans add up. Expect Biden to press him aggressively on that if moderator Martha Raddatz of ABC News does not.
Likewise, Biden and Obama have yet to explain how they intend to deal with the spiraling costs of Medicare. Expect Ryan to press Biden to offer actual solutions, instead of just scare tactics.
4 Foreign policy. The lone vice presidential debate will have nine, 10-minute sections and include both domestic and foreign policy. The latter area is where Ryan and Biden are clearly mismatched.
Romney and Ryan have stepped up their criticism of Obama's foreign policy after last month's attacks on U.S. diplomatic missions in Libya and Egypt, and the administration is facing criticism over how long it took to acknowledge the attack in Libya was a terrorist act.
Still, Biden should have an upper hand as the former chairman of the U.S. Senate Committee on Foreign Relations and as someone who played a key role in the administration's withdrawal from Iraq. Ryan has little foreign policy experience.
5 Social issues. The election is about the economy, but the Obama campaign would love to see a discussion on social issues such as abortion. That could help reverse some ominous signs it has seen lately about women voters: A Pew poll released Monday showed that Romney had closed an 18-percentage-point gap among women.
Ryan has been among the most hard-line opponents of abortion rights in Congress, including co-sponsoring a "personhood" amendment that would have made abortions illegal in cases of rape.
Romney earlier this week suggested he had no plans to pursue any abortion restrictions, prompting questions to Ryan about that Wednesday as he left St. Petersburg after two days of debate prep. "Our position's consistent and hasn't changed," Ryan said.
Incidentally, this is the first time that both tickets include a Catholic.
And it may not even be their last debate. If Obama wins re-election, both Biden and Ryan could be leading presidential contenders in 2016.
Adam C. Smith can be reached at email@example.com.