How does the Conservative leadership election work?

• A vacancy is created either by the incumbent resigning (or dying) or else by the parliamentary party passing a motion of no confidence. To force a vote on a motion, 15% of MPs (currently 50) have to write to the Chairman of the 1922 Committee requesting one. These requests can be made separately. The motion is voted on by secret ballot and a no-confidenced leader is ineligible to stand in the resulting leadership election. (For some reason, Labour friends have been very interested in this part of the rules. I can’t imagine why.)
• To stand candidates need to be proposed and seconded by MPs taking the Conservative whip; the nominations are public.
• If more than two candidates stand, there are a succession of ballots of Conservative MPs with the least popular candidate eliminated each time.
• If a ballot ties for a least popular candidate, it is held again and if it ties again both the least popular drop out.
• Candidates may also withdraw of their own free will during the rounds.
• Once the list is reduced to two, the election goes to an all member ballot across the country. To vote, members must have been paid up for at least three months and properly recorded on the party’s list.

Since five candidates are standing there’s invariably speculation about how much support is needed to get through each round. This is not the easiest question to answer. In theory a candidate needs 1/5 of MPs rounded up to get through the first round, 1/4 up for the second round and 1/3 up to make it to the final two – these figures are 67, 83 and 111 respectively.

But it gets a little more complicated as this is not a quota or transferable vote system. So if one candidate gets significantly more votes than they need in one of these rounds, it reduces the votes needed for other candidates to survive.

Conservative Home is keeping a running tally of MPs’ support for the five candidates. Using the current figures at the time of writing (adding in the candidates’ own votes) we get Stephen Crabb 22, Liam Fox 8, Michael Gove 18, Andrea Leadsom 18 and Theresa May 87 with a massive 177 MPs still to declare. On these figures May is already assured of reaching the third ballot even without attracting any more support and no-one else is certain of surviving the first.

(I stress on these figures. Past leadership contests have seen some candidates’ support drain away due to events and they’ve had the humiliation of being voted for by fewer MPs than on the running tallies.)

But because of May’s showing on these figures, it slightly reduces the target for others. The other candidates would now need only 62 MPs to survive the first ballot, though still 83 for the second. A practical example is the 2001 contest – to be sure of a place in the final two a candidate needed 56 votes on the last MPs’ ballot. But as Ken Clarke got 59, this reduced the number needed for his rival to 54 – a figure Iain Duncan Smith reach exactly, beating Michael Portillo by just one vote.

The potential for tactical voting is all too clear. If one of the candidates clearly has more support than necessary, then some of their supporters may be tempted to vote for a different candidate in order to get a more winnable final two. This possibly happened in 2005 when David Davis’s campaign was sinking and he was shedding votes between the first and second ballot. In the end he made the final two by just six votes over Liam Fox.

However tactical voting can backfire. In 2001 Michael Portillo was rumoured to have been knocked out by tactical voting by those who thought Ken Clarke would have a better chance against Iain Duncan Smith. History shows how that one turned out.