At the debate, David Cameron was out at the end of the line-up, on the far right, which is not, despite what some of his critics on the left aver, where he is usually most comfortable. On the night, however, this ought to have been an advantage. On the edge of the fray, he was in a position to wave at the pretenders to his office, turn to the camera, and raise a quizzical eyebrow - This lot? Really? It didn’t work out that way. Rather than appearing to be above it all, he seemed marginal.

Thursday was by no means disastrous for Cameron; in fact his team probably breathed a sigh of relief that Ed Miliband didn’t continue the momentum he had built up since sparring with Paxman the week before. But the Prime Minister was an oddly recessive presence, seemingly unwilling to assert his personality. After all the endless wrangling with the broadcasters, it was as if he had decided to empty-chair himself.

Cameron stuck dutifully to his key messages: long-term economic plan, don’t let Labour ruin it. Only occasionally did he attempt anything other than one of his limited but workable repertoire of expressions (<man-of-substance> face, <look-I-really-mean-this> face). Wise heads might remark that this is exactly as Lynton Crosby would have wanted; the Prime Minister simply needed this debate to pass off without incident. According to the conventional view, Cameron’s best hope of victory requires him to be as reassuring as a rock, and as uninteresting.

I’m not convinced this is right. For one thing, it’s a strategy for someone who is ahead in the polls, which the Tories are not. More pertinently, it risks amplifying a signal that I think Cameron is already sending, unwittingly. A signal that says, "I don’t want to do this job anymore".

Unlike some, I didn’t think Cameron committed a grievous mistake when he said he doesn’t want a third term. He simply answered a question honestly, something which the political classes can find hard to forgive. That is not the same as saying it didn’t hurt him, however. The problem isn’t that he made a tactical error. It goes deeper than that. The problem is that his heart isn’t in the job - and voters can tell.

Politics, like sport, runs on desire. In both fields there is a pool of people with high levels of ability and competence, and what distinguishes those who reach the highest levels – other than luck – is how much they want to get there. As Arsène Wenger once remarked, elite footballers are not highly motivated because they’re paid vast salaries; they’re paid vast salaries because they have exceptional motivation. The same is true, vast salaries aside, of politics.

One striking thing about Alistair Campbell’s memoirs is their portrait of Tony Blair. It differs significantly from the one we were familiar with when he was Labour leader: a smiling, easy-going guy. Away from the podium, Blair was an edgier and more driven presence who would think little of calling aides at 3am to discuss a policy change or rewriting a speech twenty-three times. Like Campbell’s friend Alex Ferguson, Blair was always fixated on some future goal and always worrying about how he might not reach it; each success merely brought the next opportunity for a setback into focus.

Part of the reason Blair operated at this level of intensity for so long is that he had, at his side and frequently behind his back, someone who possessed, if anything, an even more relentless drive. Having finally pushed his nemesis aside, Gordon Brown’s superhuman will to go on dragged him through humiliation after humiliation. Last week, Brown’s former aide Theo Bertram recalled how Brown had to go into the third debate with Cameron 24 hours after being pulverized by the Gillian Duffy affair. Some predicted Brown would self-destruct live on TV. But as Bertram says, he drew on “a deep reservoir of strength” to deliver a creditable performance.

You can only wonder at the Sisyphean will required to run for president, which takes at least 18 months, involves little sleep and the enormous pressure of having a big organization rely on you and you alone to perform, at or near your best, every day. The Obama-Clinton battle in 2008 was so compelling partly because it was a clash of iron wills. Or take another former presidential candidate, John Kerry, now a secretary of state with an historic deal under his belt. A diplomat told the Guardian, “A lot of the time it is not the smartest person in the room. It is question of who has the most stamina. And Kerry had an incredible amount of stamina.”

We still laugh at Thatcher for wanting to go “on and on” (Cameron used that phrase himself to encapsulate the madness of political obsessives) but we should recognize that it was precisely her excessive zeal that made her such a political giant. Politics is hard. Winning elections, and, even more so, achieving significant change once in office, requires a roaring fire in the belly.

I can’t imagine that Cameron would have displayed the bloody-minded decade-long persistence required to see the Northern Ireland peace process through. He would have given it a jolly good go, before concluding – as indeed most rational people did, over the years – that it was impossible. To characterize Cameron as lazy is itself lazy. You simply don’t get to do the job he has without a big appetite for work. But it is true that he doesn’t seem to have the exceptional drive of his predecessors.

Voters want politicians with heart for the fight. Presidential primaries are almost designed to test for it. In the town halls of Iowa or New Hampshire even the most experienced, professionally-trained, amply funded politicians can be exposed as lily-livered. We have fewer opportunities to probe for it here, but in the last two weeks we’ve had a couple, and they have been telling. Ed Miliband, on Paxman night, didn’t say anything new. He will never look like a natural in front of the cameras. But he successfully conveyed a burning desire to win, and I think that accounted for his bounce in the polls.

On that night and during the debate, Cameron has performed like an animatronic, lacking in animus. It’s sad to see. Most of us have been in jobs where we end up just going through the motions, and it doesn’t feel good.

If Cameron loses, he can reflect that it could have been worse. He could have won.