Boris Johnson’s fall is not yet a fact. But it grows more likely by the hour. The hue and cry resumed at full throttle on Wednesday morning. Then Christian Wakeford, Conservative MP for the “red wall” marginal of Bury South, who had been the regional chief of the Back Boris campaign in 2019, defected to Labour. Thirty minutes later, David Davis quoted Leo Amery (who was quoting Oliver Cromwell) telling Neville Chamberlain in 1940: “In the name of God, go.” No leader can indefinitely survive these levels of assault. Assuming it now takes place, there are two important things to keep in mind about the 2022 Conservative leadership election. The first is that the many twists in this extraordinary story may not have all been exhausted. Experience of recent Tory leadership contests suggests that we should expect the unexpected. There are many possibilities. Johnson could pre-empt a confidence vote by the uncharacteristic act of resigning. He may decide to walk away from No 10 immediately, presumably leaving Dominic Raab as temporary prime minister and boosting the justice secretary’s otherwise slim chances. One or more leadership candidates may also pull out unexpectedly, as Johnson and Andrea Leadsom did in 2016, opening the way for Theresa May’s win. Improbably, the Tory party could unite behind Rishi Sunak without a vote, as it did behind Michael Howard in opposition in 2003. The second, if the election process runs its full course, is that this will produce the first change of leader of its kind in British political history. Never before would a prime minister have been forced out by his or her party’s own MPs and then been replaced by a new prime minister chosen by the party’s members. That scenario is stuffed with political dynamite. It could be a brilliant, gamechanging success – but it could just as well be an absolute disaster. There are no precedents. One thing is sure. If Johnson loses the vote of confidence for which MPs are now sending in letters, he would certainly be out. He cannot contest the leadership election that would follow. Not even Johnson can break that rule. This is what happened to Iain Duncan Smith as leader of the opposition in 2003. But it has never happened to a serving Tory prime minister. The last three Tory prime ministers who lost power midterm all did so by resigning. Each quit because the writing was on the wall. Margaret Thatcher decided not to contest the second round of the 1990 contest in order to open the way for someone else – John Major, as it turned out – to stop Michael Heseltine. David Cameron stepped down after losing the vote to leave the European Union in 2016. Although May had survived a confidence vote the previous December, she was forced to quit in May 2019 as the Tory party fell apart over her Brexit strategy. To kick a prime minister out in a vote of no confidence is a very big call indeed. It would be a damning verdict on Johnson personally and on his “great man” view of history and himself. He might not hang around in the Commons, triggering a very losable byelection in Uxbridge. His rejection could cause him to become bitter in his old age, as Ted Heath’s did in 1975. Even more significantly, it could entrench lasting political divisions in the Tory ranks, as the fall of Robert Peel did in 1846. There would be implications for party political realignment here. The immediate political question that most matters is whether the ousting of Johnson will outrage the party membership. There is every reason to expect this, and MPs need to be prepared for it. Judging by the Tories’ recent leadership contests – this would be the third in six years – there will again be lots of candidates: not surprising, with the premiership as the prize. Eight tried to enter the race in 2016. Thirteen tried in 2019. With no overwhelming favourite this time, expect another crowded field as MPs seek to winnow the contest to the final two, from whom the membership will choose. Conventionally, political parties in revolt against themselves tend to choose the opposite kind of leader to the one they have just got rid of. Johnson rather than May is a classic example. Further back, Major rather than Thatcher is another. That might point to Rishi Sunak this time as the candidate with most of the skills Johnson lacks, or suggest Jeremy Hunt would be in with a chance. This doesn’t take into account the cultural and ideological differences between the largely southern, largely old, largely white membership and many of the parliamentarians. For that reason, the candidates who claim to be picking up the populist standard from Johnson may do best. Liz Truss is the initial frontrunner, judging by membership surveys, but Priti Patel will see this contest as her best shot too. Watch out for Steve Baker, who may decide he has the organisation skills to beat Truss among the MPs. Baker will then offer himself to the membership as the only candidate who can be guaranteed to save Brexit and prevent Nigel Farage from mounting a comeback. Much will also rest on the contest’s length and timing. The 2019 contest took nearly two months before Johnson was confirmed winner. If this one does the same, the candidates may have to juggle their responses to anything from a new Covid variant to the energy price rise, trying to sort the Northern Ireland protocol before the May elections, or a Russian invasion of Ukraine. Sunak is still probably the best bet in this race. But do not believe anyone who claims they know for certain where all this is leading. They don’t. Martin Kettle is a Guardian columnist