How runners might line up for a Tory leadership contest
The "going" - the state of the track - may make all the difference. Who takes over from Mr Cameron will, in part, depend on the innate characteristics and past performance of the contenders.
But it will also be about context, and whether the candidates' qualities seem suitable for the times. Here paradox might come into play.
The future leadership of the Conservative Party is intimately tied to the biggest decision our country has faced in decades. If the prime minister and his ally the chancellor lose the EU referendum it is widely thought that they would both have to resign.
This is indeed likely, but not certain. They might want to go and many in the party would certainly want to see the back of them. But this wouldn't quite be like losing an election.
Both sides of the Brexit debate agree that in the short term, an "out" vote would mean a degree of uncertainty, and nervousness in the markets. However much it might mark the beginning of a brave new world, there would be an air of crisis.
The track the runners and riders would be poised to race on is what American racegoers call "heavy nine" - defined as "wet track getting into a squelchy area".
It is not likely the joint resignation of the chancellor and prime minister would steady frayed nerves. They might tough it out - for the sake of the country, of course. More likely they might set a future date, maybe a year away, for their departure and an orderly contest.
But they could simply fall.
Many think the mononymous mayor has positioned himself for just such an eventuality. But Karmatic Olive - Boris Johnson (who once said he had more chance of being reincarnated as an olive than becoming prime minister) - may not suit this sort of course.
His strength is his cross-party appeal, the ability to reach parts of the electorate other Conservatives can't and don't. But while his blokeish buccaneering bravado may inspire, it may not calm nerves and soothe spirits.
Matthew Parris's devastating attack highlights just how controversial a leader he would be.
Dominic Lawson's thoughtful reaction suggests that he has Donald Trump's cartoon-character ability to emerge from an explosion with just a light dusting of debris around his shoulders. But derring-do may not be what Conservative MPs - and the country - feel like after a divisive campaign, and with awkward negotiations ahead.
On the other hand, if the going is good, the prime minister wins and can set his own timetable for departure, MPs feel the next election is in the bag - and Boris can claim to have fought the good fight.
Of course, this argument doesn't only apply to the aftermath of a referendum - Boris could be undone by any tricky economic or geopolitical background mood.
Form of fancied front-runners
Boris Johnson (51):
- London mayor since 2008, also MP; oversaw London Olympics in 2012; attends cabinet meetings, but no portfolio
- Announced in February he was campaigning to leave EU
- Charismatic, but controversial to some. Arguably the highest profile of any Conservative barring the PM
George Osborne (44):
- Chancellor since 2010, became first secretary of state in 2015, making him the highest ranking cabinet minister after PM
- Backing PM to stay in EU
- Seen politically as very clever and tactical, but not particularly likeable
Theresa May (59):
- Home Secretary since 2010, MP since 1997
- Backs campaign to stay in the EU
- Lauded for competence, regarded as safe pair of hands - can seem cold and aloof, some say
The state of the Labour Party is also likely to come into play. At the moment most Conservative MPs (in fact many Labour ones, as well) believe they cannot lose against Mr Corbyn's Labour Party. What happens to him and the party's fortunes will play into the debate.
If Labour is doing well in the opinion polls, it will change the equation. The hunt would be on for a vote winner - and Boris might be just the ticket to smash, mock and paste with Latin epitaph any potential Labour government.
But if the going is sticky, High Heels might have it. I would in any case buy shares right now in Theresa May. She's been loyal to the prime minister, without warmly embracing the "in" campaign.
Her team hasn't gossiped around Westminster, earning the ire of other senior colleagues. The Home Office is the departmental equivalent of a jungle posting and has proved the grave of many a talented politician, yet she has survived with barely a scratch.
But, although she was at the forefront of reshaping what she branded the "nasty party" she can now herself seem harsh and cold, rather dated, aloof. But if MPs are looking for a safer-than-safe pair of hands, and value competence over pizzazz, she could rise to the top.
Michael Gove is seen as a serious thinker - but neither a safe pair of hands nor electorally appealing. He might thrive if the Conservative Party was feeling extremely confident and rather bold but I suspect his role will be as an eminence grise - deputy prime minister, or chancellor in waiting - and his endorsement could prove vital.
Dark horses and new brooms
The very system of election is almost designed for dark horses. There's an "It's a knock-out", first past the post contest with only MPs voting. In each round, held on Tuesday and Thursday, the person getting the lowest votes leaves the contest until only two are left standing. The whole party then votes on those two names. For TV news this is as good a reality show as it gets.
So new brooms - the next generation - stand a chance. By definition they would benefit from the notion of an orderly succession, in calm and peaceful times.
But they also gain from MPs dithering and being unwilling to commit to a "big beast", wrongly thinking they have a free choice until nearer the end. The new Work and Pensions Secretary, Stephen Crabb, has already been endorsed by one senior party figure, and by one of the most acute observers of the party.
Nicky Morgan has made it clear she is mulling the possibility of running, Penny Mordaunt is another seen as a rising star. Remember - even a humiliating low vote raises your profile and means those who beat you come a-wooing for endorsements.
It is hard to see how any of them could make it to the final ballot of Conservative MPs without some heavyweight backing.
Down but not out
But what of Bruised Boy - the chancellor, who after his budget and IDS's resignation is seen as badly damaged goods? Napoleon liked his generals lucky, and Mr Cameron and Mr Osborne seem to have been sprinkled with more than their quota of four-leafed clovers by their fairy godmothers.
When that unlikely Samson, Iain Duncan Smith, tugged at the twin pillars of the government's reputation - fiscal rectitude and compassionate conservatism - it seemed the temple would come crashing down. But Mr Corbyn declined to give the cornerstone a little nudge with his foot.
Tory MPs shored up the brickwork with their rediscovered secret weapon and the terrible attacks in Brussels gave the media an even bigger story which drove just about everything else from the headlines for a week.
Mr Osborne has indeed been damaged and will know peril lurks around the corner, but he's still breathing. He won't think it so, but it might even help. It may not be true of horse races, but in political ones, there is no more dangerous place to be than way out in front, early on.
By being seen as limping at the back, he has time to rebuild his reputation, and one clever commentator has already suggested how it could be done.
Mr Osborne's strength was that while maybe not particularly likeable on a political level (friends say personally he is charming and fun) he was thought of as very very clever and very very tactical. This is what he has to rebuild.
And lest we forget, the chancellor is one horse who can design the track and make sure the going is good.