David Cameron can see off a challenge from Theresa May. But Boris Johnson is another matter

“We will beat the scourge of cheap booze insists PM,” said the Daily Mail headline after David Cameron tried to mask his retreat over plans for minimum alcohol pricing. Unfortunately, next to the story was an advert for cheap booze – 12 bottles of classic ales for £14.99.

It summed up the Prime Minister’s week. If there is one headline he hates, it is “Cameron in U-turn”, which to him smacks of weakness. His officials tie themselves in knots trying to avoid such headlines, as they did on alcohol pricing. As a diversionary tactic, Mr Cameron turned his fire on discounts at supermarkets. But he has a habit of making rash pledges he can’t quite deliver. He promised that people would automatically be put on the lowest energy tariffs, but the reality fell short.

His critics on the Conservative back benches are growing by the week and their mood is turning black. The party’s third place in the Eastleigh by-election did not produce immediate calls for Mr Cameron to quit but, as being beaten by the UK Independence Party sinks in, more and more Tory MPs see their leader as a loser. His failure to deliver constituency boundary changes that would have favoured the Tories has fuelled backbench fears that the party cannot win a majority in 2015. Some members of the impressive, 140-odd Tory MPs who entered the Commons in 2010 are thinking of standing down after only one term. Another common gripe is, as one influential Tory put it: “Cameron is too comfortable with coalition. He is not a Conservative Prime Minister. He is a coalition Prime Minister.”

The gloomsters’ spirits were not lifted by a presentation by Mr Cameron and Lynton Crosby, the Australian strategist who will run the Tory general election campaign. “Same old stuff on welfare and immigration,” one groaned. Mr Cameron is seen as aloof and arrogant by some Tory MPs. He knows he needs to spend more time with them and is inviting them to sandwich lunches at Downing Street in groups of 20. Yet the grumbling persists. “He was in broadcast rather than receive mode,” one complained afterwards. Another accused the PM of “going through the motions” as he “ticked another box”.

Also pressing the flesh is Theresa May, the Home Secretary. “I saw her once in the Commons tea room in 15 years; suddenly she is there all the time and invites us to ‘surgeries’ where we can raise any constituency problems,” said one senior Tory MP. “She is on manoeuvres.”

Friends say she is merely putting down a marker for the leadership race that would follow a Tory defeat in 2015, but her positioning hardly bolsters the Prime Minister. One Cameron ally said: “What does Theresa think she is up to? It’s obvious and amateurish.”

It seems remarkable that Tory MPs spend so much time speculating whether Mr Cameron could be toppled before the 2015 poll. After all, his personal ratings are higher than his party’s. When you ask the critics who they want to take over, you get a lot of “ums” and “ahs”. And yet the anti-Cameron plotting continues in a party addicted to it and which has ousted three of its previous six leaders (Edward Heath, Margaret Thatcher and Iain Duncan Smith).

At present, there are several different plots, not one co-ordinated one. One senior Tory likens them to terrorist cells, operating independently. Adam Afriyie, the party’s first black MP, is said to have “scores” of backers, but is only a potential candidate for a post-election Tory contest. If one happened before then, he would try to deliver his supporters to a bigger, more experienced beast.

The “anyone but Cameron for 2015” brigade claim it would not take much more to induce the panic that would muster the 46 Tory MPs needed to trigger a vote of confidence in him. Possible catalysts include a poor showing in this May’s county council elections; defeat by Ukip in next year’s European Parliament elections; the loss of a Tory-held seat in a by-election; damaging revelations in the trials of Andy Coulson and Rebekah Brooks and a consistent Tory showing below 30 per cent in the opinion polls.

The flaw is that a vote of confidence probably wouldn’t “kill” Mr Cameron. Even if 100 Tories voted against him, he could say he had the support of two-thirds of his MPs and carry on, just as John Major did after he called a vote of confidence in himself in 1995.

I expect Mr Cameron will lead his party into the 2015 election. But there is one scenario in which he does not. The elephant in the room of the Tory leadership debate is Boris Johnson. He professes loyalty to Mr Cameron and insists he will serve his full term as Mayor of London to 2016. Despite that, I would be surprised if he didn’t return to the Commons at the 2015 election, allowing him to enter a post-election Tory leadership race.

If Mr Johnson were offered the chance to run in a winnable by-election before 2015, I suspect he would grab it. There is even talk of one being engineered for him by a Tory backbench ally. If he won, the ever-growing ranks of the Cameron critics would finally have their answer to the “who would do any better?” question. And Mr Cameron would be in real trouble.