Andy Burnham has declared that he is the only Labour leadership candidate capable of blocking frontrunner Jeremy Corbyn and preventing a damaging split within the party.
The shadow health secretary claimed only he was able to stop the left-winger and avert a period of 1980s-style infighting.
His claims came as a poll indicated Mr Corbyn is the Labour leadership candidate most likely to damage the party's chances at the next election.
But in a sign of the left-winger's polarising effect, the poll also found he was the contender rated as having the best chance of improving Labour's fortunes.
Some 21% of those surveyed by ComRes thought Mr Corbyn would boost Labour's chances at the next election, putting him ahead of Mr Burnham on 19%, Yvette Cooper on 15% and Liz Kendall on 11%.
But the study, for the Independent on Sunday and Sunday Mirror, found 31% of those polled thought Mr Corbyn would worsen Labour's prospects of electoral success - with Ms Cooper on 18%, Ms Kendall on 17% and Mr Burnham on 14%.
The figures gave Mr Burnham a net rating of +5, with Ms Cooper on -3, Ms Kendall on -6 and Mr Corbyn on -10.
None of the candidates performed as well as former foreign secretary David Miliband, defeated by his brother Ed at the last Labour leadership contest - who scored a net rating of +11.
Among Labour voters, some 33% thought Mr Corbyn would improve the party's chances of success at the next election, with 21% saying he would worsen them.
Mr Burnham told the Sunday People: "I'm the only person in this race who can beat Jeremy."
He added: "In the 80s, we started fighting each other and left the way clear for Margaret Thatcher to bulldoze her way through Labour communities.
"I'm not going to let that happen this time."
Mr Corbyn, the frontrunner in the contest, would make the state of the British economy worse according to 36% of those surveyed, with just 14% saying it would improve if he was in charge.
Britain's standing around the world would suffer according to 37%, with just 11% thinking it would improve with Mr Corbyn in Number 10.
There was some comfort for Mr Corbyn, who has vowed to take the railways back into public ownership - with 23% believing that the quality of train services would improve compared with 22% who thought they would get worse.
Mr Burnham said: "I don't know how Jeremy would pay for all his announcements without giving cause for concern about Labour's plans for the economy."
In an indication of the task facing whoever emerges as the winner of the leadership race on September 12, the ComRes study found the Opposition trailing the Tories by 11 points, with David Cameron's party on 40% and Labour on 29%.
Meanwhile Ms Cooper launched plans to hit payday lenders with a levy in order to fund the expansion of credit unions.
In a Sunday Mirror interview she said: "Too many of these payday lenders are still ripping people off and profiting from desperation.
"It's time those lenders paid for an affordable alternative. That's why I think they should pay to support credit unions that can help people instead."
Amid growing concern from Labour MPs that Mr Corbyn is on the brink of unlikely victory, former prime minister Gordon Brown is expected to intervene in a speech on Sunday.
The ex-leader, who is held in high regard within the party and credited with a crucial late intervention in the Scottish referendum campaign, will deliver an address on "power for a purpose".
Sources in Ms Cooper's camp have played down reports that Mr Brown is set to endorse her for the leadership, but the shadow home secretary admitted she has spoken to the former prime minister.
"I have spoken to Gordon, I have spoken to lots of different people as part of the campaign, just asking people about their ideas for the future and so on," she said.
:: ComRes interviewed 2,035 adults in Britain online between August 12 and 13. Data were weighted to be demographically representative of all GB adults and by past vote recall.