Labour's crushing election defeat has given way to bloodletting, as leading Blairites turn on Ed Miliband and the Labour left, presaging a bitter ideological battle for the party leadership.
Lord Mandelson said Mr Miliband and his supporters had made a "terrible mistake" in abandoning the New Labour centre ground and undertaking "a giant political experiment" that went badly wrong.
Asked by the BBC's John Pienaar what was missing from Mr Miliband's approach, Lord Mandelson said: "An economic policy."
While many in the Labour party remain stunned by the poll result, Lord Mandelson was one of a number of Blairites who fanned out across the media to try to reclaim the party from the left.
Pat McFadden, a former business minister, said: "It would take a special kind of stupid not to learn the lessons of what happened."
Philip Collins, a former Blair speech writer and columnist, tweeted that it would take more than five years to repair the damage of the defeat: "That is the price of the Ed vanity project. He lost two elections in one night."
Labour slumped last week to its worst performance since 1987, winning 232 seats, some 99 behind the Conservatives and 26 fewer than Gordon Brown won in 2010.
The burst of activity by the Blairites was intended to stall any potential momentum behind Andy Burnham, the 45-year-old former health secretary, who is a firm favourite with public sector unions.
Mr Burnham, a state-educated Liverpudlian, has opposed the privatisation of the National Health Service and is seen by Blairites as someone who would not break from the Miliband past.
Lord Mandelson said the trade unions that helped to deliver victory for Ed Miliband in the party's 2010 leadership contest could do the same again by enrolling Labour supporting union members to vote in the 2015 contest.
"We cannot open ourselves up to the sort of abuse and inappropriate influence that the trade unions waded in with in our leadership election in 2010," he said.
Paul Kenny, leader of the GMB union, denied Lord Mandelson's claims and suggested that the Labour peer should "go back to his deckchair in his garden". He said a revival of New Labour was hardly going to win back Scotland from the leftwing Scottish National party.
However the Blairites have a problem in their attempts to stop Mr Burnham: they are not agreed on which of a younger crop of MPs is best placed to take the New Labour flag into the leadership contest.
Since Mr Miliband announced his resignation on Friday, Chuka Umunna, shadow business secretary, has signalled his intention to run, echoing the Blairite theme that Mr Miliband had abandoned the centre ground.
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> "For middle-income voters there was not enough of an aspirational offer there," he told the BBC's Andrew Marr. The shadow business secretary also rejected Mr Miliband's claim that the last Labour government did not overspend, saying "of course" the party should not have gone into the economic crash running a deficit.Bookmakers William Hill have Mr Umunna as 2-1 favourite to become the next Labour leader, ahead of Mr Burnham on 3-1 but some Blairites are not entirely convinced that the shadow business secretary has the political weight to match his undoubted communication skills.
Mr Blair wrote in The Observer that Labour had to present itself as "the party of ambition as well as compassion" and appealing "to those running businesses as well as those working in them".
But the former prime minister has not conferred his blessing on any individual candidate from the Labour right, reflecting the fact that others could claim that mantle, including Tristram Hunt, shadow education secretary.
Liz Kendall, a Blairite shadow health minister, made a case for her leadership bid on Sunday, saying the party needed to embrace ambition and public service reform.
Another contender may be Yvette Cooper, shadow home secretary, who could vie for the top job without having to deal with awkward questions about what role she would give her husband Ed Balls, who lost his seat on Thursday.
Dan Jarvis, a former officer in the Parachute Regiment and shadow justice minister, had been seen as a potential dark horse but on Sunday night he ruled out standing.
Labour's national executive committee will meet this week to draw up a timetable for the leadership contest. A shorter timetable might benefit Mr Burnham, who already has a great deal of party support, while other less well-known candidates would prefer a contest that runs into the autumn.
Harriet Harman, Labour leader since Mr Miliband's resignation, is expected to plug the ranks of the depleted shadow cabinet on Monday.
Chris Leslie, who had been shadow chief secretary to the Treasury, is expected to replace Ed Balls as shadow chancellor.
Ms Harman also needs to replace Douglas Alexander as foreign secretary - and is expected to pick a seasoned former minister from the last government. Both appointments will only be temporary until a newly elected leader chooses his or her new shadow cabinet later in the year.
Ms Harman also needs to replace other frontbenches who have lost their seats, such as Gregg McClymont, who had the pensions brief, and Tom Greatrex, who covered energy.
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