Left-wing MP Diane Abbott has promised to fight cuts to the public sector as she officially launched her bid to become leader of the Labour Party.
The backbencher also said Labour had to admit that the Iraq War was a "mistake" before it could move on.
She must get the backing of 33 MPs by 9 June to get on the ballot paper - so far only one is officially backing her, former minister David Lammy.
Ms Abbot is the only woman and only ethnic minority MP to stand so far.
She has said all the other five candidates would all make good leaders but the party "cannot be offering a slate of candidates who all look the same".
The contest was triggered when Gordon Brown resigned as both prime minister and Labour leader, when the Tories and Lib Dem struck a coalition deal following the general election.
Harriet Harman is acting leader until the winning contenders is announced in September.
Launching her campaign in her Hackney constituency, Ms Abbott said: "Within this leadership campaign, I will be the voice saying that it is not a question of the timing of the cuts, it is a question of having this level of cuts at all."
She said it was "wrong in principle that ordinary people should pay with cuts in their services and cuts in expenditure for the cost of bailing out the banks.
"When the bankers were earning the bonuses they pocketed them, so now things have gone bad, why should we pay for a crisis which was not caused by ordinary people?"
The Hackney North and Stoke Newington MP added: "Unless we have a more open debate about public sector cuts, they will do to communities like Hackney what closing down the mines did to pit villages in Yorkshire and the Midlands under the last Tory government of Mrs Thatcher.
"I believe we need to look at the balance between public sector cuts and taxation."
Responding to suggestions by Tony Blair's former communications chief that Labour would not last five minutes in power under her leadership, she said: "I say to Alastair Campbell that if the Labour Party continues with his policies, the Labour Party will never be in power again."
Ms Abbott, one of the Labour rebels who opposed the Iraq war in 2003, said she was "sad and ashamed" by the former government's controversial decision to go to war.
"We have to admit we have learnt the lessons. Until we admit these things we cannot move on as a party."
'Very best case'
Two other leadership contenders, Ed Miliband and Ed Balls, have criticised aspects of the decision to go to war, which proved divisive for the Labour Party. Neither were MPs at the time of the invasion.
According to the latest update on the Labour Party website, only the Miliband brothers have the required 33 backers.
David Miliband, the shadow foreign secretary, is in the lead with 54 MPs supporting his bid. His brother Ed, shadow energy and climate change secretary, has 45 backers.
Shadow education secretary Ed Balls has 27 MPs backing him, shadow health secretary Andy Burnham has 17 and left-wing backbencher John McDonnell has the support of six MPs.
Ms Abbott was asked whether she could get sufficent support to get on the ballot paper. She replied: "I want to make the very best case I can for taking part in the final debate over who will be leading the Labour Party. I think we are in a pivotal moment for the Labour Party."
Meanwhile Ed Miliband has backed demands for a "living wage" of more than £7 an hour. In a speech at the London School of Economics, he said: "I want to lead a Labour Party that campaigns for change in people's lives, not just for a change of government."
And in a column for Tribune magazine, Mr Balls called for the Labour Party to focus on being a "responsible and effective opposition" and stressed his commitment to Labour's historic ties with the unions.
He said Labour needed to build "from the bottom up" and reconnect with voters it attracted in 1997.