Two weeks of intense publicity have give him the look of a front-runner, but rumours of youthful experimenting with drugs and newspaper revelations about his friend and campaign manager George Osborne may undermine his standing among the 300,000 Conservative Party members in the country, even if a sufficient number of Tory MPs overlook the matter to get him into the final two. Another big question is whether, at the age of 39 and after only five years as an MP, he has the necessary experience to lead the party.
Bookies' odds: 4/7
* KENNETH CLARKE
The former chancellor is the best known of the candidates, and most opinion polls say he has the greatest appeal to potential Tory voters. But he has been in conflict with his own party for years over his pro-EU views, which are still a handicap. Some old loyalists will never forgive his role in bringing down Margaret Thatcher. It is also being said that at 65 he is too old, and he has been out of frontline politics for too long. This is more of a problem in the early rounds, when MPs are voting, than it would be if he made it to the last two.
Bookies' odds: 14/1
* DAVID DAVIS
He is still technically the front-runner, with 66 Tory MPs pledged to support him, mostly from the right but with some from the centre of the Tory party. However, his campaign may have peaked too early. There is no doubting his ambition, but some are asking whether he is up to the job. He is no better than average as a public speaker, as his address to the Conservative Party conference showed. There have been unproven rumours that his team did some persuading to collect those 66 names and that some supporters will desert him in tomorrow's ballot.
Bookies' odds: 11/5
* LIAM FOX
Fox has raised issues that are often neglected, ranging from how the party selects its candidates to care for the mentally ill. This has improved his standing, but he has been an outsider throughout, competing against Mr Davis for right-wing support without picking up notable support from any other section of the party. He could still spring a surprise if Mr Davis's support goes into free fall, but even then he could have difficulty convincing the party that he is able to attract support from voters who are not already committed Tories.
Bookies' odds: 10/1