Two of the six contenders to be Labour's next deputy leader say Labour should apologise for the Iraq war.
Harriet Harman and Jon Cruddas said they regretted voting for the war and would not have done so if they knew then what they know now.
But Peter Hain, Hilary Benn, Hazel Blears and Alan Johnson said history could not be rewritten and it was vital to focus on rebuilding Iraq.
The six were grilled by Newsnight's Jeremy Paxman in a televised hustings.
The debate also saw lively exchanges about the best way to tackle inequality, with Mr Cruddas suggesting a cap on high earners' pay could be the way forward.
Ms Harman called for a Royal Commission on the distribution of income and wealth, saying she did not a divided society "where some people struggle and others spend £10,000 on a handbag."
But Mr Johnson warned against a return to the "politics of envy", which could lose Britain investment.
The candidates were also split on the issue of an amnesty for illegal immigrants, with Mr Cruddas and Ms Harman backing the idea, Mr Johnson saying he was "attracted to it" but the other three rejecting it.
But it was the issue of Iraq which provoked some of the sharpest exchanges.
Mr Cruddas, the only backbencher in the contest, said he regretted voting for the war and he thought Labour should say "sorry" for it "as part of a general reconciliation with the British people".
But Northern Ireland Secretary Peter Hain said he did not want to "reinvent history".
"I don't think I can wriggle out of my responsibility for the decision I took honestly on the basis of the evidence I saw, which the whole world believed, which was later proved to be entirely wrong."
"I don't think because there is a deputy leader campaign on I should say I was not part of that responsibility."
Stop and question
International Development Secretary Hilary Benn said: "You can't rewind history and I don't regret the fact that Saddam is no longer in power."
Alan Johnson said 76% of Iraqis had voted for democracy "and we can not just walk away".
Mrs Blears said MPs would not have been given a vote - and the issue would not have arisen - if the intelligence had not said Saddam had weapons of mass destruction.
But Ms Harman agreed with Mr Cruddas that the only way to rebuild trust was to admit Iraq was a mistake and apologise, saying: "If I had have known there were no weapons of mass destruction I wouldn't have voted for the war."
She added: "I don't think John and I are trying to wriggle out of our responsibilities, I just think that if you are looking forward and you want to rebuild public trust and confidence you have got to admit when you have got it wrong."
There were also strong exchanges over plans to allow police to stop and question people without any grounds for suspicion.
Ms Harman expressed anger that a Sunday newspaper had been briefed on the proposals before the Cabinet, and said the government had got into "bad habits" with a "culture of spin".
Mr Hain was also strongly critical of the proposals, saying the way the information was released risked alienating minority communities who felt threatened by such measures.
But party chairwoman Ms Blears described them as "a flexible and sensible addition to the range of police powers".
Asked how they would differ from current deputy leader and deputy prime minister, John Prescott, Mr Cruddas, Mr Benn and Ms Blears all said they did not want to be deputy prime minister.
But Mr Hain said it was important the deputy leader was "at the heart of Cabinet".
Meanwhile Mr Johnson said Mr Prescott had done "a really good job", praising for the "loyal and discreet" way he had worked with Tony Blair, sometimes telling him things he did not want to hear.
Asked which of the other contenders they would vote for, only Mr Cruddas offered a firm answer, saying he would back Harriet Harman.
But despite agreeing with him on key issues, Ms Harman balked at backing Mr Cruddas, saying: "He has offered me a trade-off."
Voting forms will go out to Labour Party members, MPs, MEPs and trade unionists from 1 June, with the ballot due to close on 22 June.
The new deputy leader will be unveiled at a special Labour conference on 24 June, when Chancellor Gordon Brown is installed as the party's new leader.