A couple of weeks ago, a lousy debate performance would have finished Mitt Romney. Instead the Republican had his finest hour of the campaign, and now the roles are reversed.
A second lousy performance tonight, and Barack Obama's hopes of retaining the White House could be finished.
So passive and listless was he in Denver that people were asking the question which has cropped up periodically at low points in the first Obama term: deep down, does he even really want a second term? If the same question is being asked after round two at Hofstra University on Long Island, he won't get one.
Happily for Democrats, a repeat is unlikely. The format for the debate, Obama's own temperament, the expectations game – not to mention historical precedent – all point to a big improvement. And at the very least Obama can't get any worse. In terms of voter reaction, his defeat was the most clearcut in any presidential debate since JFK and Richard Nixon got the ball rolling in 1960.
But Obama was not the first incumbent to flop in a first debate. In 1984 Ronald Reagan appeared old, dopey and out of touch in his first face-off with Walter Mondale. "I will not make an issue of my opponent's youth and inexperience," Reagan quipped about Mondale, a former senator and Vice-President in their second debate. Reagan won re-election by a landslide.
In 2000, Al Gore recovered from a terrible performance against George W Bush in their first debate, when he came across as charmless and condescending. Though he ultimately lost the election, he won the popular vote.
Four years later and by now the incumbent, Mr Bush in his turn was in deep trouble after being outmatched by John Kerry in their first debate. But he regained his poise during the last two, and eked out a narrow win.
The second ground for hope for Democrats is their man's competitive instinct. Complacency and ring-rustiness were undoubtedly factors in the Denver debacle.
The universal panning he received will have stung Mr Obama's pride – and this is the man who in 2008 bested the strongest Democratic presidential field in recent memory, Hillary Clinton and all. Tonight, he will come out swinging, not perhaps with Joe Biden-style haymakers, but swinging none the less.
The town-hall format, where audience members pose questions directly to the candidates, should help, too. Mr Obama is a master explainer and clarifier of issues, and his relatively low-key style ought to be an asset. Town halls, moreover, place a premium on the ability to connect with ordinary voters, never a Romney forte (though he is improving).
Last but not least, the expectations game now works in Mr Obama's favour. Last time, 70 per cent of Americans beforehand thought he would win; Mr Romney's strong showing gained impact from being a surprise.
Now the Republican has raised the bar for his own performance, while any Superman aura around the President has been comprehensively dispelled by Denver.
Even so, another lousy showing will be fatal.