The battle for the Labour leadership sparked into life on Monday, as one of the architects of New Labour, Lord Mandelson, warned victory for Ed Miliband could take the party into an "electoral cul-de-sac".
Speculation was rife at Westminster that the former business secretary's intervention may be paving the way for Tony Blair's endorsement of Mr Miliband's elder brother - and main leadership rival - David.
Mr Blair on Monday recorded an interview with the BBC's Andrew Marr, to be broadcast alongside the publication of his keenly awaited memoirs on Wednesday - the day when voting papers will be sent out to MPs, trade unionists and activists for the postal ballot which will produce a new leader on September 25.
Neither Mr Blair's office nor the BBC would reveal whether the former prime minister used the interview to name his preferred successor, or is maintaining his policy of remaining aloof from the leadership debate.
Ed Miliband has used the long-running contest to urge the party to move on from New Labour, which he said had become "ideologically beached" because of its excessive caution over taxing the rich and its "control freakery".
But Lord Mandelson hit back in an interview in The Times, warning: "I think that if he or anyone else wants to create a pre-New Labour future for the party then he and the rest of them will quickly find that that is an electoral cul-de-sac."
Defending the approach that won Labour parliamentary majorities in 1997, 2001 and 2005, the peer later told the BBC: "It's an insult to the voters, in my view, to say or imply that somehow everything that they believed in the Labour Party during those elections is something that we should no longer present to them.
"David has not said that. Ed, I think, sometimes allows his rhetoric to run away from him and to allow the impression to be created that, rather than pivoting forwards from now, he wants to pivot back to some pre-New Labour stage."
Ed Miliband responded: "Peter's always entitled to his point of view and he has been a great servant of our party in the past, but I think actually now Labour needs to move on. We need to address the country and talk about the issues that matter for the country and show that we can listen to the country, and that means we need to change."
Lord Mandelson's remarks got a frosty response from some other leadership contenders. Shadow health secretary Andy Burnham - describing himself as "neither Old or New Labour, but true Labour" - said it was "time to free Labour from the grip of the warring elites trying to control our party".