Labour has been "needlessly offensive" to its core supporters by chasing headlines in rightwing papers, a deputy leadership contender has said.
Peter Hain said the party had appeared indifferent to civil liberties, and it was time to "stop insulting" the progressive majority.
Writing in the New Statesman, he said the party had lost support and its relationship with voters had soured.
The Northern Ireland Secretary is one of six contenders for the deputy post.
Mr Hain told the magazine he would not try to "wriggle out" of his part in the decision to go to war in Iraq - a reference to fellow contenders Harriet Harman and Jon Cruddas who both said last week they regretted their decision.
But he said he welcomed the possibility of a "new approach" to civil liberties in relation to anti-terrorism laws.
He said the party had been "more concerned with colluding in fantasies and fallacies" about the Human Rights Act than defending it.
Mr Hain wrote that Labour's "conscious strategy" of "flirting with the anti-liberal prejudices of the right-wing media" was dangerous, when Tory leader David Cameron was willing to embrace progressive causes.
He said Labour should not assume "progressives have nowhere else to go".
Many may choose not to vote at the next general election, because they are less worried about the prospect of a Cameron government, than about a more overtly right-wing one, he said.
"The progressive coalition that Labour so successfully assembled in 1997 has splintered because we have been careless, indifferent and, at times, needlessly offensive to the concerns and values of too many of our natural supporters," he wrote.
He added: "The credo has been that winning the centre ground - itself unquestionably vital for electoral success - is, in part, achieved by Labour defining itself against the values of progressive Britain."
That, he said, was a "false choice".
Other contenders for the deputy leadership include Hilary Benn, Jon Cruddas, Harriet Harman, Alan Johnson and Hazel Blears.
All are trying to win the support of party members and trade unionists, who have been sent ballot papers this week. The winner will be announced at a special Labour conference on 24 June.
The current deputy leader, John Prescott, will stand down with Prime Minister Tony Blair on 27 June.