Conservatives will launch a drive to "wipe out" Labour as soon as the party chooses a new leader, one of the candidates has warned.
Liz Kendall said the Tories would "throw everything at us" after the leader is named on September 12 and called on the party to "get real" about the threat to its future and the need to choose someone who can win the 2020 general election.
Her comments, made during a live televised Sky News debate with the other three contenders, will be seen as a plea to members not to elect left-winger Jeremy Corbyn, who her supporters believe would hand David Cameron an opportunity to seize control of the centre ground.
An unscientific survey of 12,000 viewers for Sky found a large majority felt Mr Corbyn had won the debate, with 80.6% naming him, against 9.1% for Ms Kendall and 5.7% for Yvette Cooper and just 4.6% for Andy Burnham. This marked a sharp increase in support for the Islington North MP over the course of the show, after almost 67% of those voting said at the outset that they wanted him to win.
Mr Corbyn won loud applause as he called for Labour to reject the Conservatives' austerity programme and ensure that the poorest and most vulnerable were not made to pay for the mistakes of the bankers.
But Ms Cooper told him he was "offering people false hope" with his plans for more quantitative easing to fund investment.
"Once the economy is growing, if you simply keep printing money, that pushes up inflation and that money still has to be paid back", she said, insisting that Labour must offer a "credible" economic alternative to Conservative cuts.
Mr Corbyn retorted that Labour should not go into the 2020 general election promising more cuts, insisting: "I say, invest to grow."
Mr Burnham warned that Labour had "drifted away" from people in England and Scotland, who saw it as too "London-centric" and switched to Ukip or the Scottish National Party. He said Labour had failed to offer a clear alternative to austerity at the election because it was unwilling to propose tax rises, and had "lost sight of its principles" in opting to abstain in a vote on cuts earlier this year.
He warned that Labour would be making a "terrible mistake" if it indulged in infighting following the leadership election, rather than maintaining its fire on the Tories.
And Ms Kendall warned: "Let's get real. The Tories want to wipe us out, and the minute the new leader is elected they are going to throw everything at us.
"They are trying to take our politics, claiming they are the One Nation party and the party of the low-paid. We know they are not, but they know they won't be re-elected if they are still seen as the party of the privileged few.
"They want to take our money and our funding from trade unions ... and they are going to change the constituencies, reduce the number and do it on individual voter registration, which they are bringing forward by a year. They are going to whip up English nationalism, aided and abetted by the SNP.
"They want to wipe us out and we've got to get real, we've got to go back on the attack against the Tories. They are going to bring it on and we need a strong Labour leader who understands what they are going to do and is going to fight back for Labour so we can win in 2020."
The candidates split over their views of the legacy of former leader Tony Blair.
Mr Burnham said Mr Blair had "made mistakes" but it was "ridiculous" for Labour supporters to direct their anger at a man who had delivered three general election victories for the party.
But Mr Corbyn said that he "fundamentally" disagreed with the former prime minister over Iraq. Despite some achievements during his time in power, Mr Blair "made such a tragic mistake that his history will always be the question of Iraq and the dishonesty that went with it," he said.
Mr Corbyn said that Britain should be "adult and grown-up" about cannabis and decriminalise its use for medical purposes, though he said he would not advocate decriminalisation for recreational use "at this stage".
Mr Burnham said there was "a case" for allowing cannabis to be taken by sufferers from some conditions who feel it eases their symptoms, but warned: "It would have to be highly regulated in order to allow it for medicinal use."
Both Ms Cooper and Ms Kendall suggested that it should be subjected to the usual trials imposed on any substance thought to offer medical benefits.
Mr Corbyn said he would rule out support for any extension of British military action against Islamic State into Syria, warning of the danger of "mission creep" leading to the deployment of ground forces and insisting that the answer to the civil war lies in a political agreement in the region.
Ms Kendall challenged him: "Are there any circumstances in which you would deploy Britain's military forces?"
The left-wing MP paused briefly before replying: "Any? I'm sure there are some, but I can't think of them at the moment."
Asked whether he stood by comments quoted in The Sun which appeared to suggest he believed Britain's armed forces should be reduced, Mr Corbyn replied: "I don't know what the remarks are because I don't buy The Sun newspaper.
"I do think we have to think about the level of armed expenditure we have in this country - £35 billion per year. We are in the top five of military spending across the whole world. I think we have to seriously look at those issues and look at the issue of nuclear weapons as well.
"Can we afford to have global reach from a country of 65 million people off the north-west coast of Europe? Should we not be more interested in supporting international law, working with the UN, rather than deciding that we as quite a small country can afford this global role?"