Conservative Party rivals David Davis and David Cameron have each expressed confidence they can win the leadership.
Mr Cameron said Mr Davis was "a formidable politician"
Mr Davis told the BBC's Sunday AM show he was "fighting to win" and that polls showed he was gaining on his rival, rising 18 points in five days.
But Mr Cameron warned a Tory government would be more likely if he was leader.
He said the party needed to change: "If we play the same tunes, we end up with the same song, we'll end up with the same position in the charts - second."
On Friday, ballot papers were sent to the 300,000 Conservative Party members around the UK. They have until 5 December to cast their votes.
The candidates now go before a nationwide series of hustings.
Time for a change
Mr Davis acknowledged he was trailing in the polls but said he was picking up support and believed he could still win in the ballot.
Conservative Party members were deciding who was going to be the next Tory prime minister, he said.
He believed they would focus on "experience, toughness, resilence, principles and strategy" and that support for Mr Cameron was "softer" than polls suggested.
"If you believe the polls... we've moved 18 points in five days, that's not bad going and I expect that to continue as Tory party members - many who have not decided yet - focus on what this is about," he continued.
Mr Davis said it was time for a change in political direction as they came to the "end of the era of Blair."
"What I'm saying is 'now is time for a different approach, a principled approach, a direct approach, what you see is what you get approach'."
He warned "image-led" politics was dangerous.
And he reiterated his desire to cut tax but said the Tories would have to convince voters they were prepared to do this.
Appearing separately on the same programme, Mr Cameron said planning future tax cuts now, was a "misjudgement".
"What we can't do is try and write the whole of the 2009 manifesto in the next few weeks," he said.
"I think setting out tax plans £38bn of tax cuts... to start in four years time and to finish in nine or ten years time. I just simply don't think is sensible."
Mr Cameron said Mr Davis's "right wing core vote" strategy would lead to defeat at the general election.
"We need this intellectual revolution in the party to really do the long-term work that will help convince people that the Conservative Party has understood why it has lost," he said.
He would give a clear sense of direction during the leadership campaign, and bring in the "best brains" to carry out policy work.
Mr Cameron also defended his call for the party to consider downgrading Ecstasy from a class A to a class B drug.
He said he did not underestimate the dangers of Ecstasy but the government's drugs policy had been a failure.