Labour leadership frontrunner Jeremy Corbyn would take the party "backwards" and risks leaving Britain a one-party state, David Miliband has warned.
In the latest of a string of interventions from the party's big beasts, the former foreign secretary endorsed Liz Kendall to take the top job and disclosed that his second preference vote would go to Yvette Cooper.
The "angry defiance" of the Corbyn campaign will lead only to electoral defeat, he warns in an article for The Guardian.
Mr Miliband wrote: "Given the collapse of the Lib Dems, the stakes now are very high indeed, not just for Labour but for the country.
"Get it wrong, and Britain could become a multiparty democracy with only one party - the Conservative Party - that can win parliamentary majorities. A one-governing-party state."
Mr Miliband dismissed the radical left-winger's "demand that Labour become an anti-austerity movement on the Greek model".
He added: "The alternative to Syriza/ Corbyn is based on passionate reform, not angry defiance. Clear-eyed about the dangers of inequality, the corrosion of the public realm, the weakness of international political cooperation relative to economic forces, the necessity of domestic institutional change, it does not seek to fight the next election as a party of angry protest but instead as one of reforming government.
"The Corbyn programme looks backwards. The pledges of nationalisation, 7p in the pound increases in national insurance for those earning more than £50,000, and equivocation about Britain's place in the EU are the same ideas that I learned were wrong when I joined the Labour party in 1981."
It comes after both Mr Burnham and Mr Corbyn offered to "involve" each other in their teams if they succeeded Ed Miliband.
Mr Burnham told activists in Manchester there is a "good deal of common ground" between him and his rival on some of the major policy areas, including transport and education.
As he reached out to the Islington North MP's supporters, the shadow health secretary said Mr Corbyn had brought "real energy to this race" and he would "involve" his rival in his team from the outset.
But Mr Burnham warned there is a "real risk of division" following the increasingly bitter leadership contest and insisted he was the one candidate who could unite the party.
He said it was crucial for Labour to have a "credible" plan for public finances and criticised the Islington North MP's opaque stance on Britain's future in the European Union as well as his plans for renationalisation of utilities and "printing money" to pay for infrastructure.
Speaking at the People's History Museum in Manchester, he said: "My worry is that policies like these would leave us open to losing the argument on the economy on day one."
He added: "I won't let our party repeat the history of the early 1980s when we were more interested in fighting each other and we left the pitch clear for Margaret Thatcher to bulldoze her way through Labour communities up and down the country.
"I won't let those Bullingdon Boys - the heirs to Thatcher - do the same to people in 2015 as she did in 1985.
"That was the year I joined Labour here in the North West. I have given my life to it ever since.
"I am now fighting to win this contest with everything I've got because the very future of our party hangs in the balance."
After the speech, Mr Corbyn said: "We welcome Andy's inclusive tone towards our campaign and the view is mutual - if we win we would involve Andy in our team if he was willing. From day one, whoever wins must pull the party together."
Meanwhile, Yvette Cooper and Liz Kendall have denied reports that Lord Mandelson approached them in a bid to suspend the election by persuading the pair and Mr Burnham to drop out en masse.
While Ms Cooper said she had not been approached directly by the grandee and said she was unaware if her campaign team had been contacted, Ms Kendall insisted that neither she nor her office had spoken to the peer.
Asked about the claims made by the Telegraph, Ms Kendall told BBC Radio 4's World At One: "No, neither me nor anybody in my team (have spoken to Lord Mandelson). I have no idea where that came from."
Ms Kendall also rubbished suggestions that she has been asked to drop out of the race to boost Mr Burnham and Ms Cooper's chances of winning.
She said: "I haven't made that proposal and none of the other candidates have made that to me.
"I have said that I want people to vote for me with their first preference but they should use their second or third preferences for Yvette or Andy or Andy or Yvette.
"I'm not going to be dropping out in this contest, I can't stop making the case."