Conservative leadership candidates have attacked each other's policies days before the party announces the results of its ballot on leadership rules.
David Davis's agenda would be a 'lethal injection' party, critics said
According to Ken Clarke's prominent backer John Bercow, David Davis' right-wing agenda would be a "lethal injection" into the party.
However, choosing Mr Clarke to lead would be a form of "national suicide," said rival David Cameron.
Changes to the rules could give MPs the final say, rather than party members.
Shadow education secretary Mr Cameron referred to Mr Clarke's support of Europe - an issue which may have scuppered his two previous leadership bids, but which he insists will not do so this time.
Mr Cameron told GMTV: "If we go down a path where we say that the European issue somehow doesn't matter any more and we're not going to engage in that, I think instead of political suicide, that's national suicide."
His comments were dismissed by Mr Bercow as "ludicrous hyperbole".
He added that Mr Cameron's combination of "Eton, hunting, shooting and lunch at Whites" made him the wrong man for the job. Mr Davis also came under criticism from Mr Bercow.
Sleepwalking to slaughter
"If we just go on mouthing the same old mantras [about Europe]...my fear is that effectively what we will be doing is administering a lethal injection," he said on GMTV's Sunday Programme.
"We will be sleepwalking to a fourth successive slaughter and deservedly so."
Mr Clarke remains confident of becoming the next Conservative leader, despite having failed in two previous bids.
He said he was the most popular Tory with the public and in any other country that would make the contest a "walkover".
Asked if he thought he would end up as leader, Mr Clarke told ITV1's Jonathan Dimbleby programme: "I think I will actually."
Outgoing leader Michael Howard has pledged his full support for his successor but refused to give his backing to any candidate.
Meanwhile, shadow foreign secretary and fellow leadership rival Liam Fox has said the tax burden on hard-working people in Britain must be reduced.
He said he would not rule out a flat rate of tax - a policy rejected as "too right wing" by Mr Clarke.
And he called for an "English Parliament" to decide on issues devolved from Scotland.
He also told BBC's Sunday AM programme that he said what he thought and was not a "mealy-mouthed politician".
Under a flat tax, householders and businesses would pay the same rate of tax on all their earnings.