What happened? How was a President of such uplifting image and such a way with words given such a drubbing by a man reckoned by even his own supporters to be a poor campaigner, especially when it came to connecting with ordinary people?
Yet by near unanimous consent, precisely that happened on Wednesday night in Denver. Barack Obama was listless and lacklustre, unwilling for whatever reason to take the battle to Mitt Romney. Maybe he considered it un-presidential. Maybe he simply wasn't in the mood.
He made no howlers to be sure, in a contest notably short of memorable lines. But he let Mr Romney, in debating terms, get away with murder. He didn't press him on specifics. He didn't mention the famous 47 per cent of Americans who, his opponent said, consider themselves "victims". He didn't mention Bain Capital, 14 per cent tax rates, Cayman Islands investments or the other "B-Word", as in George W Bush. Mr Obama delivered a rambling lecture. "Let's talk about taxes because I think it's instructive…," he droned at one point. Mr Romney was often mendacious, but pithier and more pointed.
There are precedents for this dismal showing by an incumbent. An ageing Ronald Reagan badly fumbled his first debate against Walter Mondale in 1984 (though he made up for it in the second, promising he would not "take advantage of my opponent's youth and inexperience.").
Eight years later, George H. W. Bush finished a distant third in his first debate with Bill Clinton and the independent candidate Ross Perot, who was generally adjudged the winner.
Then in 2004, Bush Jnr was well beaten – not only in the first, but in all three of their debates – by his Democratic opponent John Kerry (who played Mr Romney in Mr Obama's practice debates; evidently not that brilliantly.)
One common factor was lack of practice. Like any president, Mr Obama has spent his last four years giving speeches, holding rallies and generally having people jump at his every command. But the stage in Denver featured no re-assuring presidential seal, reminding Mr Romney and all America who was boss. On Wednesday evening, Mr Romney was an equal, and gave at least as good as he got. "Probably no-one's talked to [Mr Obama] like that since he won the White House," one observer noted.
Mr Obama was also a victim of the expectations game – not that he could have done much about that, given how Mr Romney had been written off in advance after his gaffes and sliding poll ratings. As the former Massachusetts governor showed during the primary campaign, he can be a decent debater. But by Wednesday the assumption was that Mr Obama would deliver a knockout blow. Pre-debate polls showed two thirds of Americans expected the President to win.
And that in turn may have bred complacency; that all he needed to do was show up. The President "wanted to have a conversation", said James Carville, a key operative in Bill Clinton's winning team in 1992. But "it takes two people to have a conversation. Mitt Romney came in with a chainsaw." Coming from one of the most ruthless Democratic chain-saw wielders of recent times, that was praise indeed for the Republican.
More worryingly perhaps for his supporters, Mr Obama's limp performance fitted into a pattern. He is prisoner of his reputation as an inspirational orator. In fact, his manner is that of the professor, the careful observer, the lucid synthesizer, driven by logic rather than passion. The danger though is that detachment becomes drift.
And it's happened before during his presidency. On occasions during the health care wars he was oddly passive, allowing a disputatious Congress to dictate events. Just before the Democrats' mid-term debacle, some party officials privately wondered whether Mr Obama's stomach was still in the fight, whether he even wanted a second term.
But he did – and there's every reason to expect he'll fight back now. Anyone who watched him defeat Hillary Clinton in their epic 2008 primary battle (or has seen him play pick-up basketball) knows how competitive he is.
Two more debates lie ahead and it would be amazing if Mr Obama passed up so many opportunities to score points. And as John Kerry proved, winning debates is no guarantee of winning election. But the prospect of an Obama walkover is no more. As David Gergen, CNN analyst and veteran of Republican and Democratic White Houses, put it: "We've got a horse race."