Labour leadership hopeful Andy Burnham has revealed he would serve in the shadow cabinet if left-winger Jeremy Corbyn takes the party's top job.
The shadow health secretary made the admission hours after polling put the veteran backbencher as the shock front-runner in the race to succeed Ed Miliband.
Yvette Cooper left the door open to remaining on the front bench but moderniser Liz Kendall ruled out taking a position with a flat "no".
During a leadership hustings on LBC, she warned that a Corbyn win would put the party out of power for "a generation".
"I think it would be disastrous for the party, it would be disastrous for the country, we'll be out of power for a generation," she said. "I don't want to be a party of protest and I wouldn't be able to stop myself from making that case."
Pressed whether he would take a shadow cabinet position under Mr Corbyn, Mr Burnham said: "I would. I would serve the Labour Party at any level it asked me to serve the Labour Party."
Ms Cooper said: "I'd be torn because the truth is I think it would be quite hard given the differences in terms of policy.
"The other thing is I don't think you should walk away from the Labour Party."
The debate came at the end of a fractious day for Labour as its factions clashed over the future direction it should take following its woeful general election performance.
Tony Blair issued a stark warning to the party not to repeat the mistakes of the 1980s which consigned the party to 18 years in opposition.
The former prime minister said a shift to the left after the party's crushing general election defeat would be to treat voters as if they were "stupid".
He derided Mr Corbyn as the "Tory preference" and said the party could not regain power if it was simply a "platform for protest" against cuts.
"It would not take the country forwards, it would take it backwards," he said. "This is why when people say 'My heart says I should really be with that politics' - get a transplant."
Research by YouGov for The Times found Mr Corbyn was the first preference for 43% of party supporters - way ahead of bookies' favourite Mr Burnham on 26%, while Ms Cooper was on 20% and Ms Kendall 11%.
During the radio hustings, the contenders were put on the spot by Ukip leader Nigel Farage, who phoned as "Nigel from Kent" to challenge the Labour candidates over whether they could ever see themselves voting No in the referendum on EU membership.
Liz Kendall said she was a Yes voter "first and last and always" while Ms Cooper said: "If I thought it wasn't in Britain's interest, I would vote No, but I think it is in Britain's interests."
Asked if there were circumstances in which he might vote No, Mr Burnham said: "There aren't, because I always will believe that being in Europe is better for jobs in our country."
Pressed by the other candidates to say whether he could ever vote No, Mr Corbyn said that "if Europe becomes a totally brutal organisation which treats member states in the way it has treated Greece", then it would lose the support of many people.
Mr Corbyn was the only one of the candidates who said he would bring Mr Miliband into his shadow cabinet.
The Islington North MP also emerged as the only contender never to have taken drugs, saying he was "really boring".
Ms Kendall said she "had a few smokes" when she was at college but preferred dancing and having a glass of wine as ways to relax.
Mr Burnham admitted to indulging "once or twice" at university while Ms Cooper said she had tried cannabis as a student.