Joe Biden has to salvage the Obama campaign
The vice-presidential debate is often thought of as a sideshow, perhaps like the moment of light entertainment in a circus when the clowns come on between the high wire acts.
But there is no place for custard pies tonight. This one matters.
The first presidential debate has shaken up the race. President Barack Obama puts his lame performance down to being "too polite".
There are some more just out, from swing states, that show the same thing.
It is important not to overstate the importance of one moment, and of one week's polls. But right here, right now, it does seem as if the Denver debate could matter quite a lot.
So Joe Biden has to come out swinging to give Democrats some new sense of hope and purpose. Paul Ryan has to punch hard to build on Mr Romney's success.
The debate promises to be both fascinating and substantial.
Hero versus the clown?
Paul Ryan is the young conservative hero because he has come up with a plan to cut America's spending which even some opponents respect because it sort of adds up (although one early admirer has changed his mind).
It also means some pretty severe cuts and would change the way those who are now middle aged get healthcare and pensions. His aim has to be to come over as cool and reasonable and to provoke Mr Biden.
Win or lose, many see him as the future of his party, so he has a lot to prove, and a lot to lose.
By contrast, many within the Washington beltway see Joe Biden as a bit of a joke, undisciplined and loose-lipped.
Maybe. But his bluntness can be appealing to blue-collar voters even as it appals buttoned-up strategists. His speech to the Democratic convention was funny, emotional, loyal and above all an effective appeal to the white working and middle classes.
His aim will be to show that Republican figures either don't add up, benefit the very rich or mean cuts that would hurt America. He is under a lot of pressure to think on his feet and to deliver a series of killer one-liners while showing restraint and discipline.
It could be quite a show.