Five more Tory MPs announced they would vote for Mr Cameron, including a former cabinet minister, Peter Lilley, who suggested that Mr Cameron's ability to ward off smear attacks over alleged drug abuse had strengthened his claim on the leadership.
"It has demonstrated that David Cameron doesn't waver under fire. He hasn't been exposed to this kind of thing before and he has held his own very sensibly," Mr Lilley said. "If we all are going to require every potential candidate for the leadership or the prime ministership to go through all the seven deadly sins and say they have never committed them, then all we will choose is somebody who is good at lying."
Kenneth Clarke announced the names of two new supporters prompting MPs to speculate that the right-wing candidate Liam Fox was the most likely contender for elimination in today's ballot. But the guesswork was clouded by doubts about the so-called "lie factor" - MPs who have pledged to support one candidate in public but may vote for someone else in secret.
The candidate most vulnerable to the lie factor is David Davis, who has public pledges from 66 MPs, and whose campaign could run into severe problems if he does not achieve that figure today. Mr Cameron has the promised support of 39 MPs, Mr Clarke 26, and Dr Fox 22. Of the 45 MPs who have not declared, about half may not have made up their minds and the others were refusing to say.
MPs will be voting all day in a secret ballot that will reduce the number of candidates to three. A second ballot on Thursday will eliminate another candidate, and the final two will go to a ballot of 300,000 Conservative Party members.
Mr Clarke's campaign team was urging party officials in the country to ring MPs today to make sure that the former chancellor gets through to the final two. Mr Clarke said: "I sense that what most people want is for David Cameron and myself to go forward."
Mr Clarke will be under intense pressure to pull out and back Mr Cameron if he comes third in today's ballot, but his campaign organisers were adamant that he would fight on.
"He came third last time and he could still come top," said a member of his team.
Alistair Burt, another Clarke backer, said: "If colleagues leave the most popular Conservative in the country off the ballot paper, Conservative supporters will wonder what we are about."
Mr Cameron's problems about his alleged drug use were still dogging him yesterday. A BBC Newsnight poll suggested that 28 per cent of voters would be less likely to vote Conservative if the party were led by someone known to have taken cocaine - although 66 per cent of the 1,003 adults polled by ICM said it would make no difference.
In an apparent dig at his rival, David Davis told London's Evening Standard that it was "absolutely right" that the Metropolitan Police should target middle-class drug users.
But Mr Cameron was able to turn the speculation about his past to his advantage when he addressed a packed meeting of MPs and peers in the Commons last night. The meeting, organised by the backbench 1922 Committee, was addressed by each of the four candidates.
Mr Cameron began by saying: "I have had a week when I have been tested and you have seen how I dealt with that", before moving on to wider questions of policy and party organisation.Reuse content