Peter Hain has warned Labour that it must adopt a more balanced approach to civil liberties if it is to win back the "middle-class progressives" who have deserted the party.
In an interview with The Independent, he set out a four-point plan to woo disenchanted voters who abstained or backed the Liberal Democrats at last year's general election - many of them over the Iraq war.
The Northern Ireland Secretary, who is to contest the election for Labour's deputy leadership, will argue that he is the man to "galvanise" the lost voters whose protest handed 30 seats to the Tories.
His manifesto will call for: a softer approach on individual liberties; a much stronger commitment to green issues; replacing the first-past-the-post electoral system (without first holding a referendum) with peers being elected at the same time as MPs; and a major devolution of power to local government.
Mr Hain said: "We should be a party that is not just as tough as nails on security but also jealous of protecting individual civil liberties. I think we need to take a lot of care about ensuring we get the balance right in the future," he said. His remarks will be seen as expressing concern about Tony Blair's plans to rebalance the criminal justice system in favour of victims and extend "summary justice" such as fixed penalty fines. He added: "We must be a party of good government and progressive government, not 'big brother' government and the nanny state." He will urge Labour not to instinctively propose bans.
Mr Hain will offer MPs and party members a genuine partnership with the Government "rather than lecturing and bouncing them and telling them about new policies on the front pages of newspapers". He believed people would then feel "a sense of ownership" about policies instead of being disenchanted with the Government. "But this is not about fudging or avoiding hard choices and difficult decisions."
He is backing Gordon Brown for the leadership. Although he would welcome a "contest rather than a coronation", he believes that Mr Brown is "unbeatable", describing him as "the towering figure of this political generation in all parties along with Tony Blair.
"I think he [Mr Brown] will grow into the job. People will have two years in which to see him as an outstanding Prime Minister."
Asked whether Mr Brown should face a cabinet-level challenge, he replied: "People will make their own minds up. I don't think you unify the party by frogmarching candidates into a contest. You have got to have people who want to stand, who are entitled to stand and who take their chance."
He is worried, like many Brown supporters, that the attacks on the Chancellor from within the Labour family are harming the Chancellor's public standing. "I think it's time people let the party members decide and not try to pursue their own personal, selfish and factional objective from any quarter," he said.
"I have never been a Blairite or a Brownite," he said. "I have had my differences with both Tony and Gordon in the past. I think that puts me in a better position than many to say that I genuinely believe - just as Tony has been the most successful Labour Prime Minister ever - so Gordon will be a more than worthy successor."
Mr Hain claims he has the support of 45 Labour MPs, one more than the number of nominations required to enter the deputy leadership contest.
But his strong backing among the trade unions may be undermined by the entry into the race of Jon Cruddas, the MP for Dagenham.
Mr Hain is refusing to bow to union demands to repeal some of the Thatcher government's industrial relations laws, including a ban on secondary action. He will warn the unions that the Cruddas plan for the deputy leader to be outside the Cabinet would deny them a voice at the heart of government and would be "not a recipe for partnership but a permanent stand-off " with the deputy leader "shouting at the Government and Prime Minister from afar".
He paid a warm tribute to John Prescott, saying he had been a "totally loyal" deputy to Mr Blair and Labour's achievements in the past nine years could not have happened without him. Mr Hain believes the deputy party leader should also be Deputy Prime Minister, as Mr Prescott is. "We have established a precedent which is a good one," he said. But Mr Brown is keeping his cards close to his chest about that and may have other ideas.