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.