Andy Burnham pledged to fight a "positive" campaign as he hit out at Tony Blair for warning that the Labour Party faced "annihilation" if it picked Jeremy Corbyn as its next leader.
The former frontrunner who has seen his left-wing rival shoot ahead of him in the battle to succeed Ed Miliband also criticised the "pretty appalling" behaviour of Tory MPs and other rivals seeking to exploit membership rules.
But he insisted he welcomed the arrival of tens of thousands of new supporters to the ranks of voters despite fears of "entryism".
In a stark intervention, Mr Blair wrote in The Guardian that the party he led to three successive general election victories was "walking eyes shut, arms outstretched, over the cliff's edge to the jagged rocks below.
"This is not a moment to refrain from disturbing the serenity of the walk on the basis it causes 'disunity'. It is a moment for a rugby tackle if that were possible."
Asked if he agreed, Mr Burnham told the Press Association: "Everybody is entitled to express their views on this contest, particularly former prime ministers.
"But I don't think it helps necessarily to second guess the outcome and to make these dire predictions on the back of it.
"I will be keeping it positive. I want to get my case over to the 600,000 people who have a vote in this contest.
"They will rightly decide the future of the Labour Party now. I think people are well aware of the issues at stake and I think the time has come to trust the members of our party and the supporters of our party to make the right decision about its future."
Amid controversy over the election process, he said it was "not the time to cry foul on the rules".
"It should be a cause for celebration that the Labour Party has the ability to bring 600,000 people into its leadership election.
"I think it is a positive thing that there is a vibrant contest and that there are a wide number of people from different walks of life able to have a say."