Boris Johnson just significantly increased his chances of becoming Conservative Party leader

Thinking about career options. PA/Stefan Rousseau

Having electrified Brexit backers with his decision to campaign with them and against the government ahead of the EU referendum, Boris Johnson instantly suffered a full-on prime-ministerial assault. At their first appearance in the House of Commons since Johnson tooks sides, David Cameron went on the attack.

Johnson would have known that in breaking with Cameron and a large majority of the cabinet, he was painting a target on his back – and the first shots have already hit home.

In assuring the House of Commons that “I am not standing for re-election. I have no other agenda than what is best for our country”, Cameron appeared to be accusing Johnson of doing the opposite. The implication was that Johnson had become the standard-bearer of the Leave campaign in order to improve his chances of victory in the next Tory leadership contest.

Johnson’s father rejected the claim that his son was motivated by career considerations, saying that the decision could be “career-ending” if the Brexit camp loses the referendum.

How to win a leadership contest

An important variable in any leadership election is the selection system that a party uses to choose its leader. This was clearly demonstrated when Jeremy Corbyn won the Labour leadership contest on a wave of support from grassroots members despite having minimal backing among MPs. It was an all-member ballot and the voice of the wider membership prevailed over the inner circle.

The Conservatives have a somewhat different selection system. Both MPs and party members participate but in different stages of a two-stage process.

When a Conservative leader resigns – which would presumably be the circumstance in which Cameron would depart – candidates, who must be MPs, come forward and can enter the contest for succession provided that they have their nomination papers signed by a proposer and seconder, both of whom must be MPs.

If there are three or more candidates, there follows a series of secret elimination ballots among Conservative MPs. Each MP votes for one candidate and the candidate with the fewest votes is eliminated. The parliamentary ballots continue until the field has been whittled down to two candidates. Their names will then go forward to a ballot of individual party members in the country.

Whichever candidate wins a majority of party members’ votes becomes the leader. This system, which was introduced in 1998, was used to select Iain Duncan Smith in 2001 and Cameron in 2005.

One of the interesting features of this system is that, while the winning candidate would need to have demonstrated broad support among MPs, they would not require majority support from them, only from the party members.

The first goal is to finish in the top two in the parliamentary ballots, even if that is a long way behind the candidate who tops the poll. That can have major consequences if there is some ideological difference between MPs and party members – as there was in the Labour Party last year.

Leader or loser?

Evidence is sketchy and so we must be careful when drawing conclusions. However, a 2015 poll found that 71% of Conservative Party members wanted to leave the EU. Only 24% wanted to remain, while 5% didn’t know. We do not yet know how Cameron’s renegotiation will affect those figures.

This suggests that even though the leavers and remainers in the parliamentary party are fairly balanced (the latest figures show 142 Tory MPs want Britain to remain in the EU, 120 wish it to leave, and 68 have not declared), the membership itself is strongly eurosceptic.

If Conservative MPs alone chose their leaders, as they did before 1998, the contest to succeed Cameron would be a very tight call (if it were shaped primarily by the EU vote). If MPs were looking for someone to unite the party after a fractious referendum, a leading leaver would probably not be the obvious choice.

But under the Tories’ selection rules, the parliamentary ballots do not serve the function of picking the leader but of presenting the membership with a choice. Candidates do not need to appeal to all wings of the parliamentary party, just a sufficiently large group of MPs to finish in the top two. Given the strong eurosceptic leanings of the membership, Johnson might feel confident of winning the membership ballot against a remainer. He just needs to get on that ballot first.

He may have just increased his chances of doing so. With the exception of Johnson, all of the figures usually described as the leading contenders to replace Cameron are remainers. The chancellor, George Osborne, was always expected to support the remain side. There was speculation that the home secretary, Theresa May, could back Brexit but, in the end, she too sided with remain.

Splitting the vote? PA/Anthony Devlin

Other possible candidates, such as the foreign secretary, Phillip Hammond, or outsiders like the education secretary, Nicky Morgan, or business secretary, Sajid Javid, are also supporting remain.

The only cabinet heavyweights to back leave are the justice secretary, Michael Gove, and the work-and-pensions secretary, Iain Duncan Smith. Neither seems a plausible successor to Cameron.

That just leaves Johnson to mop up eurosceptic support within the parliamentary party. With Osborne and perhaps May splitting the pro-EU support between them in a leadership contest, Johnson’s decision to campaign to leave may have cleared his way to the all-member ballot stage. If he had supported remain, he could not have counted on the gratitude of eurosceptic MPs and may have struggled to distinguish himself from, and elevate himself above Osborne and May, both of whom have impressive experience as ministers.

These calculations may hold, regardless of the result of the referendum. If Britain votes for Brexit, Johnson becomes the obvious successor to Cameron, who would presumably resign immediately. If the country votes to remain, Johnson would not necessarily have damaged his chances, provided that he was seen to have fought a good fight.

He would still hope to benefit from the support of eurosceptics within the parliamentary party and the membership at large, and as a leaver, might be more trusted to ensure there was no backsliding by the EU on delivering Cameron’s negotiated agreement.

Johnson was once seen as the likeliest successor to Cameron, but that was at a time when the Conservatives’ electoral prospects were unpredictable and the state of the economy precarious.

With Labour’s shift to the left, and a strong economic upturn underway, Osborne became the heir apparent and Johnson looked more of a risk for his party, when a risk wasn’t needed. To re-establish himself as a serious candidate to replace Cameron, Johnson had to take a risk of his own. In campaigning to leave the EU, he has done that. Time will tell if the gamble pays off.