The three main runners each have strengths and weaknesses. Unfortunately for the Tories, they can't have the best bits of them. Allies of Mr Davis, the front-runner, are convinced he is the man with the "X factor" to woo voters. His critics fear he might prove a divisive leader who would not broaden the party's appeal beyond its core vote, the trap into which William Hague, Iain Duncan Smith and Michael Howard all walked.
Backers of Mr Clarke say he is the candidate Labour fears most, the one best-placed to exploit the economic storm clouds the Tories hope will gather before the next general election. Opponents say the 65-year-old former Chancellor is past his sell-by date and doubt his hunger to be Leader of the Opposition, the toughest job in British politics.
Conversely, critics of Mr Cameron, 38, believe he is a young man in too much of a hurry. While his inexperience may make him a bigger risk than Mr Davis or Mr Clarke, aides insist that his election would offer the biggest upside: it would dramatically show the voters that the Tories had changed.
The entry of Mr Clarke into the leadership stakes - he will announce his intention to run within a week - has certainly enlivened the process. There will be a battle between him and Mr Cameron to emerge as the standard-bearer for party modernisers who will take on Mr Davis on the right.
Mr Clarke can be too laid back for his own good. Until recently, most Tories believed his prospects of leading his party had long gone. Now, suddenly, many think he has one last chance. The former Chancellor has allayed the doubts of those who feared his appetite for the leadership was considerably less than his appetite for a good lunch. "He is up for it this time: he didn't really want the job in 2001," one MP explained. That was news to many who campaigned for him then, but never mind.
I am told the Clarke strategy will be simple. He will rise above the Tories' policy debates and present himself as the one man who can defeat Gordon Brown at the next general election. So his fire will not be directed at his Tory opponents but at the man who succeeded him at the Treasury.
Mr Clarke is also tempted to play the Iraq card, to make a link between the war and the London bombings. Some are telling him to leave the issue well alone, that making the link did not do Charles Kennedy much good, even though most MPs admit privately that the war is to bound to have increased the threat to London.
Other Clarke admirers argue that he must highlight his good judgement in opposing the Iraq war.
Although some of Mr Clarke's natural allies have already signed up for Mr Davis or Mr Cameron, partly because he did not tell them he would stand, there are signs of a more concerted Clarke push than in 2001. Last time he rejected pressure from aides to trim his pro-European views. This week, he was happy to say that British membership of the euro was off the agenda for 10 years and that the European constitution was dead.
His rivals believe the party will not buy his tactical move on Europe, saying the issue is not dead, just sleeping, and will return. Mr Clarke's biggest weakness may be that he will offer fewer forward-looking policies than his opponents.
He has an infuriating habit of answering policy questions by saying: "When I was Home Secretary/ Health Secretary/ Education Secretary/Chancellor..." One Tory MP, who subscribes to the view that the party needs to offer some "10,000-volt shocks" on policy, says: "The trouble is that Ken thinks he is the 10,000 volt shock."
Although Mr Clarke and Mr Cameron are different political animals, they are fishing for votes in the same pool and so it would make sense for them to combine forces. This looks unlikely because neither man wants to play second fiddle. They had a frosty meeting in June when Mr Clarke went to Mr Cameron's House of Commons office (a tactical error, perhaps). The Old Pretender found the Young Pretender rather arrogantly (in his view) asking for his support. Mr Clarke expected Mr Cameron to back him.
The Clarke strategy now is to eclipse Mr Cameron in the hope that the shadow Education Secretary withdraws to fight another day. But Mr Cameron is being told by friends not to wobble; that he, not Mr Clarke, is the only candidate who can beat Mr Davis.
Cameron allies accuse the Davis camp of talking up Mr Clarke's prospects because it fears him a lot less than Mr Cameron. They are convinced that Mr Cameron, a moderniser rather than a Tory left-winger, is much closer to the party's centre of gravity than Mr Clarke, and say that he would not necessarily back Mr Clarke if he were eliminated from the contest.
As Mr Clarke prepares for his last hurrah, it is not clear how long it will last. It could be over in a few weeks. Yet he could lead the party into the next election. No one knows. What is clear is that he will shape the Tory contest and that we will enjoy his final roll of the dice.