What sticks in my mind from the last few days is the image of a fellow journalist at a news conference given by the Prime Minister in a Cambridgeshire car park. As David Cameron spoke about the importance of Britain’s membership of the European Union, my colleague was listening on his headphones to Ken Livingstone on the BBC’s Daily Politics explaining why he thought it was important to talk about Hitler’s early policies towards the Jews.

Cameron cannot believe what is happening. The mood in Downing Street is far from triumphalist. Instead it is a mixture of incredulity and dismay. The Prime Minister needs a functioning Labour Party to help to get the vote out in the referendum. With no other votes on 23 June, turnout may be low and local organisation will be important. Because most Conservative Party members support the Leave campaign, the only organisation with get-out-the-vote experience on the ground to which the Remainers can turn is Labour. The party’s demoralisation over the past five days can only have made it slightly harder for the pro-EU cause to win.

The phone polls give Remain a solid lead, an average of 55 to 45 per cent, but Cameron is nervous. Internet polls, including ORB for The Independent yesterday, have it 50-50. I am persuaded that phone polls are more likely to be accurate: online panels tend to have too many Ukip-minded respondents on them. But you can see why the Prime Minister is worried.

He is like all party leaders. A surprising amount of their time is spent working out how they might lose power and trying to prevent that happening. Gordon Brown had a healthy contempt for those plotting against him. He thought they were a bunch of shambling amateurs, and so they were, but they could have brought him down. For the Tories, William Hague was paranoid about Michael Portillo, who was indeed out to get him.

That is why Cameron still spends a whole morning preparing for Prime Minister’s Questions. Despite appearances of going for a walk in the park, I’m told he still feels a “shudder” at 10 minutes to 12 every Wednesday. He doesn’t have to be afraid of Jeremy Corbyn to know that it is still important to do well on such a closely observed occasion.

His immediate concern is about the referendum. He knows that if he loses it, he will be gone, although Matthew d’Ancona recently wrote that there is a plan to put off his actual departure for rather longer than the “30 seconds” predicted by Kenneth Clarke.

What surprised me, however, in a recent conversation with a top source in Number 10, was how insecure Cameron appears to be about what happens even if he wins the referendum. I had thought that talk about an attempt by Tory MPs to depose him if the people vote to stay in the EU was a delusion of the irreconcilables. But I’m told that the Prime Minister doesn’t rule it out.

It would need 50 Tory MPs – 15 per cent of the parliamentary party – to write to the chairman of the 1922 Committee, Graham Brady, to demand a vote of confidence in the party leader. That “could” happen, Cameron thinks, even if he thinks he would win the vote of confidence that would follow.

Even a failed attempt to get rid of him would be damaging. It would also be pointless, as he’s going anyway before the next election. But one of the consequences of Labour’s disarray is that the leash of electoral discipline has been taken off the Tory dogs.

The huge question, if Britain votes to stay in the EU, is how the Tory party responds. If the result is decisive, say more than 60-40 for Remain, it is possible that the Tory Outers will knuckle under and go back to complaining about the foreign aid budget. Anything closer than 55-45, though, could result in the Outers declaring a moral victory and intensifying the struggle, SNP-style. And that could cut off Cameron’s leadership long before his personal target date of 2019.

I have written before about the inevitability of Boris Johnson. Recently there has been a bit of a backlash against him. Tory MPs are not rallying to his camp in the way they did to Brown and to David Davis, with different results, in previous succession struggles. But the commentators who say Johnson has made a big mistake in coming out for Brexit are those for whom the idea that any intelligent person might sincerely conclude that Britain would be better off out of the EU is outlandish.

I am told that Cameron accepts that the party will want an Outer to succeed him. “Their hearts beat in that direction,” he says. And there isn’t another Outer apart from Johnson who could be leader. More to the point, there isn’t an Inner either, even assuming the party could accept the result of the referendum. It’s hard to see how George Osborne can come back from deep unpopularity this time. Theresa May doesn’t seem to want it. Nicky Morgan has reverted to Education Secretary type by annoying teachers and parents. Stephen Crabb is just starting on the vote-repelling universal-credit brief.