Mitt Romney will probably make up ground on Barack Obama tonight. What a terrifying thought!

Of the myriad distinctions between the human being (homo sapiens) and the politician (homo egomaniacus), the clearest concerns speaking in public. You and I fear it like little else, which is why glossophobia features in every list of the top 10 phobias with arachnophobia (spiders), acrophobia (heights) and estherclaustrophobia (being confined in a broken lift with Esther Rantzen).

Terrible things befall a mind presented with even an invisible audience (as a radio presenter long ago, I introduced a guest with a jaunty: “And now, it’s a pleasure to welcome to the studio, to chat about his new book... I’m so sorry, I’ve completely forgotten your name”). With an audience in plain sight, it is much worse, of course, and anyone who ever made a wedding speech will recall the stomach-churning torment of the minutes preceding the clinking of glass.

So try to imagine how it feels today to be Willard Mitt Romney, hours before the first presidential debate in Denver, contemplating his final chance to save his candidacy not only in front of a silent crowd of a few hundred in the arena, but an audience of 50-60 million TV viewers at home. Even accounting for the axiomatically deranged self-belief of anyone who thinks he should be leading the free world, it must be petrifying enough for Mittens to risk catatonia with an overdose of Xanax.

Now it may be that the debate’s power to decide the presidency is overrated, and only three from history are cited as game-changers: Nixon’s sweaty top lip in 1960, Gerald Ford denying any Soviet domination of Eastern Europe in 1976; and Al Gore’s contemptuous sighing at Dubya in 2000. But this election could yet be a photo finish, like that trio, if Romney understands that the words he speaks are infinitely less significant than how he projects himself.

The same goes for a party conference set piece like Ed Miliband’s effort of yesterday. Very few people listen intently to the content of such a keynote address, if at all, and even fewer lines (“The lady’s not for turning”; “A Labour council – a LABOUR council”) are remembered the next day. What does sink in – from an amalgam of body language, timing, delivery and tone – is a sense of a candidate’s fitness to lead. With Iain Duncan Smith and his quiet-man drivel, you concluded that he might as well go the whole hog and have an elective laryngectomy. With Mr Tony Blair’s post- 9/11 kaleidoscope-in-flux ravings, it became clear that the Messiah complex needed emergency treatment from a psychiatric crash team.

There is nothing remotely sociopathic about Ed Miliband, as he confirmed in Manchester with a fluent and mildly engaging speech. Whether the faux-chilled meandering across the stage, or clumping juxtaposing of his parents’ escape from the Nazis with his childhood love of Dallas, suggested a normal guy is a different matter. His hand movements were off-puttingly forced. The account of his small son telling him to talk about the dinosaurs, combined with his reliance on the appellation “Friends”, hinted sadly at Ross Geller, sitcom history’s nerdiest nebbish. Presentationally, Miliband is improving, and the confidence is growing. But playing normal remains a shade beyond his actorly range for now.

Whether or not Mitt Romney, the phoney’s phoney, does much better tonight (tomorrow 2am our time), it would be no surprise if he is deemed to have performed surprisingly well. Having cunningly lowered expectations with the most self-contradictory campaign since Popeye ran for Congress on the Spinach Prohibition platform, many viewers will be startled by the sharp, effective counter-puncher who twice destroyed Newt Gingrich, a strong debater himself, in the spring. Mittens, who has been rehearsing for months, will have an overflowing arsenal of well-rehearsed “zingers” to deploy against Obama, an average debater who has before regarded churning out sound bites as beneath his dignity. It is a moment – perhaps the last moment – of genuine peril for the Prez.

If it revolved around foreign policy, it would suit Obama better, and not merely as Slayer of Bin Laden. Only the nutters on the American far right want war with Iran, and it is the likelihood of President Romney waging one in alliance with his old friend Bibi Netanyahu that makes it so crucial that he loses.

This first debate concerns domestic affairs, however, and with most US economic indicators remaining dreadful, it favours Romney, even though he is desperately vulnerable himself as the flippiest of flippers and floppiest of floppers. God knows how such a snobbily robotic apology for a presidential wannabe means to persuade the 47 per cent that he loves them. But if he can begin to do that – and understands that his mission is not to land Wildean haymakers, useful though they are, but to hide the weirdness and project himself as a faintly normal guy – he could transform Obama’s stroll back to the White House into squeaky-bum time.

Whatever the result of this debate, Romney will remain the outsider, but a deemed victory will mean he will have far more chance tomorrow than he has today. That is such a truly terrifying prospect, with Iran in mind, that we must prepare to be in on the birth of a new morbid terror, Mittensophobia. It would make speaking in public strike even the young George VI as the giddiest of fun.