From heir apparent to meek exit in one chaotic morning. After a week of post-Brexit political turmoil, former London mayor Boris Johnson’s decision not to seek the Tory leadership is arguably the biggest surprise of all.

Having led the Leave campaign in the EU referendum, the assumption was that Johnson would sweep to victory in the contest to replace David Cameron as Conservative leader and prime minister. Instead – and once again – the favourite in a Tory leadership election has failed to land the prize. In this case, he hasn’t even contested it.

How can we make sense of these developments? Two general features of leadership contests help to explain: the criteria that selectors use to choose candidates, and the rules of the selection process.

Academic research has shown that three criteria weigh heavily in party selectors’ minds when choosing a leader. First and most important, candidates must be broadly acceptable to their party and able to unite it. That is especially important when a party is clearly divided on an important issue. Divided parties rarely win elections.

Second, candidates should look able to lead their party to electoral victory. Third, candidates should possess the competence required to make an effective prime minister.

Johnson’s greatest presumed strength was his electability. It was he, after all, who won the London mayoralty twice in a Labour-leaning city. Questions had been raised about his competence for the role of prime minister in comparison with other candidates, such as Theresa May, who has served as home secretary for six years. Boris has no cabinet experience.

But what finally appears to have torpedoed Johnson’s chances is the criterion of acceptability. In the first place, there were considerable worries that Boris would not be able to unite the Leavers and Remainers in the Conservative parliamentary party. He has become increasingly seen as a divisive figure who antagonised Remainers with his attacks on the prime minister during the referendum campaign.

He now looks to have become unacceptable to growing numbers of Leavers, too. There has always been a suspicion about Johnson’s motives in campaigning for Leave. Some believed he was only doing it to hasten his own chances of becoming Tory leader. Certainly, his conversion to the Leave camp came late in the day. Leavers’ fears may have been deepened by a recent article in the Daily Telegraph in which Boris appeared to have cooled on Brexit. If someone is seen as untrustworthy, it damages their credibility.

How the field shapes up

The Conservatives’ selection rules were initially thought likely to favour Boris, but then appeared to work against him. In this system, MPs take part in a series of secret ballots to whittle down the candidates to two. These then go forward to a vote of 150,000 party members.

When Boris announced that he would campaign for Leave in February, the assumption was that he would scoop up the support of most Leave MPs while allowing the two principal Remain candidates, George Osborne and Theresa May, to split the Remain MPs between them. Boris would be guaranteed a place in the top two and would win the final ballot of a eurosceptic and grateful party membership.

Osborne’s hopes imploded after his disastrous u-turn on tax credits and his perceived scaremongering during the EU referendum. That left May as the main hope for Remain MPs, although her detached attitude during the campaign and her refusal to become involved in mudslinging have helped her to appeal to some Leavers as a unity figure. Another Remainer has joined the fray in the form of Stephen Crabb, the work and pensions secretary, although he is seen as an also-ran at the moment.

The danger to Boris would have been if a credible Leaver emerged to compete with him for the votes of Leave MPs. That would risk splitting the Leave vote in the final ballot of MPs, when there were three candidates left, handing first place to May, while Boris struggled with a rival Leaver to finish in the top two.

Liam Fox, a prominent Leaver, announced his candidacy but looks unlikely to do well, given that he left the Cabinet several years ago. Another Leaver, Andrea Leadsom, the energy minister, has also entered the contest, but she too looks a long-shot. Boris would have been confident of defeating both in the parliamentary ballots and attracting their support as they were eliminated. If one had reached the final three candidates, overhauling Boris to finish in second place would have been a tall order.

It was the surprise intervention of justice secretary Michael Gove, a leading Brexiter, that changed everything. He is a much stronger proposition for Leave MPs than either Fox or Leadsom and a more plausible alternative to Johnson.

Gove’s standing grew during the referendum campaign and unlike Johnson, he has long been a committed eurosceptic. It was long assumed that he would strike a deal with Johnson and offer his backing. Few thought he would stand for the leadership himself.

Then, just before Johnson was due to announce his candidacy on June 30, Gove issued a cutting statement:

I have repeatedly said that I do not want to be prime minister. That has always been my view. But events since last Thursday have weighed heavily with me. I respect and admire all the candidates running for the leadership. In particular, I wanted to help build a team behind Boris Johnson so that a politician who argued for leaving the European Union could lead us to a better future. But I have come, reluctantly, to the conclusion that Boris cannot provide the leadership or build the team for the task ahead.

Apparently referring to concerns about Johnson’s suitability to be prime minister and his potential to backslide on any deal, Gove decided to shoot for the top job.

Several prominent MPs, including some previous backers of Boris, quickly announced their support for Gove. It can only be the case that Johnson suffered a haemorrhaging of support within the parliamentary party and no longer felt confident of finishing in the top two. His shock withdrawal from the race shows how damaged he has been about the questions over his acceptability.

May now looks the favourite to succeed David Cameron, although Gove will fancy his chances of finishing second in the parliamentary ballots. There would then be all to play for in the membership ballot. He has already grown in authority in one campaign this year. Can he now assert himself in another?

Questions might be raised about Gove’s suitability, given his previous declarations that he did not want to become Tory leader. There will also be concerns about his trustworthiness after he spectacularly turned on Johnson and destroyed his leadership hopes. However, MPs and party members will likely pay close attention to opinion polls to determine which of the candidates plays best at the box office with the voters. Electability will be a factor.

One thing is for certain, though: this is already one of the most astonishing Conservative leadership campaigns ever – and it’s hardly even started. What other surprises are in store between now and September?

Tom Quinn, Senior Lecturer, Department of Government, University of Essex

This article first appeared on The Conversation.