The Labour leadership frontrunners have been booed and jeered by union activists as they refused to condemn Tory plans for a £23,000 benefits cap.
Shadow health secretary Andy Burnham, shadow home secretary Yvette Cooper and shadow health minister Liz Kendall indicated they did not oppose the limit in principle during a hustings organised by the GMB in Dublin.
The rough ride came as the three confirmed their places on the ballot paper for the contest by recording support from at least 35 MPs. Figures published by Labour show Mr Burnham has been formally backed by 53 Commons colleagues since nominations opened this morning, ahead of Ms Cooper on 40 and Ms Kendall on 36.
Left-winger Jeremy Corbyn has been endorsed by 11 MPs, and shadow international development secretary Mary Creagh just five. Nominations are open until June 15.
The candidates faced questions over the Labour-union link, MPs' pay, the benefit cap and the party's election manifesto at the hustings.
Everyone except Ms Creagh said they would not share a platform with Prime Minister David Cameron in the run-up to the promised referendum on the EU. Many believe part of the party's implosion in Scotland was down to Labour politicians joining Tories at meetings and rallies during the independence referendum last year.
Ms Kendall said the party was under "mortal threat", adding: "The scale of the defeat means we must change or face irrelevance."
Mr Corbyn, the most left-wing of the candidates, told delegates his purpose in standing was to raise issues about how to face austerity and the way the Government will treat people over the next five years.
Mr Burnham said Labour had lost touch with many supporters and was seen as a "Westminster elite", talking in "political code".
"I will take Labour out of the Westminster bubble and will lead a party that people can relate to," he said.
Ms Creagh said Labour was trusted to run the NHS, councils and schools, but parts of the electorate did not trust it to run the economy.
"Labour needs a fighter - and I am a fighter," she said.
Ms Cooper said Labour's election campaign was too "narrow", adding that the party should reach out to win back voters "left behind".
Mr Burnham and Ms Cooper were heckled by delegates for failing to give a yes or no answer when they were asked if they backed Conservative plans to reduce the cap on household benefits from £26,000 to £23,000 a year.
Both Ms Kendall and Ms Creagh said they were in favour of the change - to ensure it was impossible to receive more in welfare payments than the average family earned from work, though they warned it could cause localised problems.
To cheers, Mr Corbyn was the only one to declare himself opposed, insisting that in the absence of regulated private rents it would result in "social cleansing" in central London.
But while Mr Burnham said he backed the principle of the cap, he insisted it was "unfair" to expect such a direct answer.
"I am not setting my face against changes to the benefit system but it depends how they do it and I am not going to give you an answer like that to a question that complicated," he told Mirror journalist Kevin Maguire, who was chairing the session.
Ms Cooper also faced dissent from the floor as she declined repeated requests to answer yes or no, despite saying the current policy was "unfair" and warning of damaging implications.
"I understand that everybody wants a yes or no, but we need to reform the legislation. As it stands, I think it is unfair.
"It is our job in the Labour Party to try and argue for change and to try and make changes, whether that is in Parliament or anywhere else and that is what we are going to have to do."
All five candidates said they did not believe Labour's election manifesto was too left wing, with Mr Burnham praising former leader Ed Miliband for the policies the party fought on.
Asked by Scottish delegate Duncan Walker how Labour could win back votes in Scotland, all five agreed the party needed to re-connect with voters.
Ms Cooper said Labour had to be a party for the whole country, adding: "We have to rebuild - but it will take us time."
Mr Burnham said: "I would say that it is the best manifesto that I have stood on in the four general elections I've stood for Parliament for Labour.
"I pay tribute and give credit to Ed Miliband. I believe he did something important in re-focusing our party on inequality."
Ms Creagh stressed the need for major investment in the transport infrastructure.
"We had a huge row about High Speed Two. I want High Speed Three, Four and Five. I want Crossrail Two, Three and Four. Physical mobility is key to social mobility," she said.
North West and Irish delegate Kevin Flanagan asked the candidates - to loud applause - if they were in the toilet when Labour's election manifesto was drawn up.
Mr Burnham said he was not as involved as he would have liked.
He repeated that he had decided not to accept any donations from unions for his campaign because he believed it would put him in a stronger position to defend the Labour/union link.
In contrast, Ms Creagh said she would be happy to take donations from unions.
Ms Kendall said she would have liked to see the manifesto contain pledges to help elderly people.
Mr Corbyn said there was an issue about democracy in Labour since the role of the party's annual conference been reduced in favour of policy forums.
Ms Cooper said many policies in the manifesto were good, such as the planned changes to zero hours contracts, and pledges to scrap fees for taking cases to an employment tribunal.
The five candidates were asked a series of specific questions to test their knowledge of the price of goods and services.
Ms Creagh correctly answered how much a TV licence cost, saying she paid by direct debit; Mr Corbyn thought a prescription in England cost £7.60, rather than the correct figure of £8.20; Ms Cooper thought the minimum wage for apprentices was under £3 an hour (it is £2.73).
Mr Burnham incorrectly guessed a litre of petrol was £1.60 rather than £1.16 and Ms Creagh knew how much she paid for a loaf of bread (£1.25).
Mr Corbyn received the loudest applause in the hall, such as when he pledged to attend an anti-austerity rally in London on June 20, but delegates said later there was no stand-out winner during the two-hour-long hustings.
The result of the election will be announced on September 12.