Pat McFadden MP was absolutely right to argue today that if Labour only opposes cuts there is a "danger of being tuned out by the electorate". But in many ways, the electorate has tuned Labour out already and the challenge is to tune it back in.

Being credible on the deficit is now a hygiene factor for Labour's next leader. All the candidates, and Labour's frontbenchers, are leading opposition to the unfairness of the Budget and the cuts to come in October's Spending Review. This is politically necessary, but not politically sufficient. Labour will get elected again, not on the determination of its opposition, but on the credibility of its alternative.

The perceived wisdom of the 1992 shadow Budget is that setting out proposals in opposition for necessary tax rises -- or indeed, spending cuts -- is political suicide. But it was a significant move this week from Ed Balls that started this debate, after he told the BBC's Laura Kuenssberg that it was a "mistake" for Labour to promise to halve the deficit in four years.

It was a major scoop for her, but did he mean to go public with this view? He hasn't yet said what his alternative would be and so has left himself a credibility gap for the other candidates to probe at the next hustings, in London this Friday.

The dilemma remains: how does Labour prove credibility without being explicit about either alternative cuts or tax rises? It almost certainly can't be done.

At the end of last week, Ed Miliband said Labour shouldn't be locked into the 2:1 ratio for cuts and tax rises, as stated in Labour's manifesto. He said the new leader should outline cuts by the time of October's Spending Review and suggested that tax rises play a bigger role. Andy Burnham has since joined the call for more tax rises and has been bold and consistent in arguing that raising the NHS budget while cutting the social care budget is a mistake.

These are more forward-looking attempts at gaining distinction and they do help move Labour on from what felt, at the time, like a credible deficit reduction plan on page 6 of Labour's manifesto. Page 6 was the core script for all Labour spokespeople during the election, and in interview after interview, Ed Balls and others made it sound credible. Having lost an election so dominated by the Tory campaign against a National Insurance rise, Labour now needs a new deficit reduction plan that is not politically tarnished.

David Miliband has suggested doubling the banking levy and introducing a "mansion tax". Ed Balls would start the 50p income-tax rise at £100,000 and Ed Miliband would make it permanent. Yet all the candidates are struggling for credibility, because these tax rises don't add up to the spending cuts they are seeking to prevent and the VAT hike they voted against last night. Demos has costed an alternative, but it is not the only option.

McFadden is right that the Tories and Lib Dems want Labour to "retreat to its comfort zone" so that they can argue Labour is responsible for the deficit and that "they alone are capable of facing up to Britain's problems".

Labour's next leader needs to win two arguments at the same time. The first is that the deficit was not mismanagement, but a decision taken to prevent recession turning to depression. The second is that Labour's new leader can now be trusted to reduce it. The public will not accept one without the other.

Richard Darlington is head of the Open Left project at Demos.

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