Normally, vice-presidential debates hardly matter. On 6 November, Americans choose a president, not a vice-president. Who remembers a word from Cheney-Lieberman 2000, Cheney-Edwards 2004 or even Biden-Palin 2008? But tomorrow's match-up between Joe Biden and his Republican challenger Paul Ryan is different.
Thanks to President Obama's flop in last week's face-off with Mitt Romney, the stakes are suddenly much higher. Mr Biden can't single-handedly rescue his boss with a strong performance in Danville, Kentucky – but a lousy one could convince undecided voters that this Republican ticket is a risk worth taking. In which case, the startling pro-Romney swing after the first presidential debate would become harder for Mr Obama to reverse in their two remaining encounters.
Expect Mr Biden to venture into areas Mr Obama inexplicably steered clear of in Denver: Mr Romney's personal tax arrangements, Bain Capital, his now disavowed dismissal of 47 per cent of Americans as "victims," as well as Mr Ryan's controversial budget plan, passed in March by the Republican-controlled House of Representatives.
As Chairman of the House Budget Committee, Mr Ryan is the top Republican economic policymaker on Capitol Hill, and his plan, slashing government spending, cutting taxes and part-privatising Medicare, offers Democrats a much clearer target than the vague proposals of Mr Romney.
So the debate is potentially make-or-break for Mr Ryan. His choice as vice-presidential nominee sent a jolt of excitement through Republican ranks. This is his chance to show his stuff – and also reassure Americans that he is up to the Oval Office, should President Mitt Romney fall under the proverbial bus.