Now that the Tory party has confirmed we’ll know the identity of its new leader and therefore in theory our new prime minister in the week beginning 22 July, it is also possible to capture the single issue that will dominate both the coming two weeks of voting by MPs – who will choose the shortlist of two – and then the definitive choice by party members. It is this deceptively simple question: will Boris Johnson save or sink the Tory Party?

Right now the former foreign secretary and Churchill biographer is streets ahead of the pack, both in respect of the declared support of Tory MPs and popularity among party officials and the membership.

Now to be clear the big doubts are not about his competence, even though some think that is a proper concern, given that those who worked with him at the Foreign Office, in City Hall (when he was London Mayor) and at The Spectator Magazine (where he was editor) don’t unanimously sing his praises.

But although there are questions about his diligence and judgement, his vulnerability – and the implied messages of the rival campaigns of Jeremy Hunt and Michael Gove – is not what he is but what would happen were he to become prime minister.

Because they both think that if Johnson becomes PM, there will inevitably be a general election in the autumn – and before Brexit is delivered.

And their sotto voce argument is that for all Johnson’s formidable skills as a campaigner, his record of winning two mayoral elections and the EU referendum, even he could not rescue the Tory Party from wipeout if there were a poll this year.

Johnson is aware of the concerns – because this is what he told Tory MPs tonight:

“We are facing an existential crisis and will not be forgiven if we do not deliver Brexit on 31 October. I believe I am best placed to lift this party, beat Jeremy Corbyn and excite people about Conservatism and Conservative values.

“We need to realise the depth of the problems we face. Unless we get on and do this thing [Brexit], we will be punished for a very long time. There is a very real choice between getting Brexit done and the potential extinction of this great party, but I believe I can take on Farage and win back the voters being won over by him.”

Johnson also told the Tory One Nation group’s leadership hustings that he would not wish to call a general election.

So his challenge is to persuade his colleagues that an election would not happen whatever his own preference, or that if there were to be that election he could prevent his party being wiped out.

So why might his accession to the leadership of party and country trigger an election? Well, were he put on the Tory crown a small number of ultra Remainy MPs – Grieve and Gyimah come to mind, for example – might well abandon the Tory party and vote with the opposition to bring down the government. Not that it matters so much, but quite a few Tory lords are also muttering about resigning from the party rather than serve Johnson.

And even if Tory Boris sceptics were to stick by him for a few short weeks, Johnson’s commitment to getting out of the EU by 31st October probably means leaving the EU without a deal – because there is probably more chance of me winning Miss World (if it still exists) than of EU leaders giving Johnson a Brexit deal they refused Theresa May (and especially before a new EU Commission is constituted on 1st November).

Since there is equally close to zero prospect of MPs not doing everything it takes to prevent a no-deal Brexit, and probably succeeding, Johnson would then face his ultimate Hobson’s choice, of asking for a no-deal mandate from us, from British citizens, either in a referendum (which I am not sure that he ruled out tonight, but if not that would presumably be oversight rather than ruse) or in a general election.

So I am now going to reframe the question faced by those choosing the next Tory prime minister: is Johnson remotely credible when he says he can take the UK out of the EU by 31st October without triggering a general election; and if he is not credible on that, are they frightened or exhilarated by the prospect of him facing off against Farage and Corbyn?

Tory MPs now have six weeks to ruminate on that. The fate of the country hinges on what they conclude.

Robert Peston is ITV’s Political Editor. This article originally appeared on his ITV news blog