Thursday 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 Adam C. Smith's five things to watch.