The Conservatives will lose the next general election if they pick David Davis as leader, rival challenger David Cameron has claimed.
David Cameron has criticised his rival's plans
Mr Cameron said the Tories faced a choice between shoring up their core vote or reaching out to new supporters.
Both contenders have previously said the Conservatives could win the next election under either of them.
Mr Davis' campaign said Mr Cameron's attack showed he was "rattled" by Mr Davis' progress.
In a speech on Thursday evening, Mr Cameron tore into Mr Davis's plans to cut tax and create new grammar schools.
"If all we have to say on the economy is up-front personal tax cuts, what do we have to say to the millions of people in our country who don't pay income tax but who are still living in poverty? "If all we have to say on education is `Bring back 20 grammar schools', what do we have to say to the parents of children in the 26,000 schools where standards are falling and Labour are failing?"
He also pledged to bring an end to the "Blair era" in British politics.
"Our party faces a clear choice in this leadership contest.
"Do we move to the right, or do we fight for the centre ground ? Do we stick to our core vote comfort zone or do we reach out ? Do we repeat the mistakes of the past, or do we change to win for the future?"
"That is our choice. If we get it right we can end the Blair era. If we get it wrong, the Blair era stays - under new mis-management: more spin, more headline-chasing, more bureaucracy, more centralisation, more spending, more borrowing, more waste, less reform, higher taxes - from Gordon Brown."
A senior source in the Davis said Mr Cameron's attack on Tony Blair was a sign he was "rattled".
"The fact is he has based his whole campaign on portraying himself as the heir to Blair. "Everything he and his supporters have said and done suggests they are entranced by Blair.
"It is clear that David Cameron himself would be Blair Mark II and we won't end the Blair era by aping Blair."
On Thursday, Mr Davis told BBC 2's Newsnight he thought the Conservatives could win an election with Mr Cameron at the helm.
And he said he did not accept claims that he was known as a "thug" and a "bully" by political colleagues.
Interviewer Jeremy Paxman said Mr Davis's colleagues had used offensive language when asked their opinion of him.
He said Mr Davis had been dubbed "an adventurer, disloyal, congenitally treacherous and winner of the whips office shit of the year".
Mr Davis replied: "What you are getting there is a view from a small group, I stress a small group." He admitted upsetting colleagues to get things done but "disputed every one" of the descriptions.
He also defended his claim he would cut taxes by £38bn a year by the end of his parliament if he became prime minister.
Asked if this was a promise, he said: "This is the strategy ... there will be a reduction in taxes to create growth in the economy."
Pushed further on whether he was guaranteeing to reduce taxes by this amount, he said: "I did not use the word guarantee, I said it was a strategy."