Just as their morale was rising along with Labour's poll ratings, party chiefs have been stunned by Brian Sedgemore's defection to the Liberal Democrats.
And it appalled Labour candidates in marginal seats faced with the tricky task of talking round voters considering switching support to Charles Kennedy's party.
Lord Kinnock, who was Labour leader from 1983 to 1992, spoke for dozens of them as he tore into Mr Sedgemore for damaging the re-election chances of people he once called comrades.
"What I'm most concerned about is the feelings that will be engendered among his colleagues of three or four weeks ago from the Parliamentary Labour Party fighting marginal seats," Lord Kinnock said.
"For them, his statement will appear to be a lance right through the spine."
David Blunkett, the former home secretary, joined the counter-attack, warning voters they could be handing power to "disaffected" Labour MPs if they followed Mr Sedgemore's exhortation to "give Tony Blair a bloody nose".
Mr Blunkett underlined the fears of Labour's high command that a collapse in its majority could hand influence to its "awkward squad" of MPs.
Dismissing Mr Sedgemore's call to voters to deny Labour a big majority as stupid, he said: "People in marginal seats can make an enormous difference to whether a government has a sufficient majority to lead the country. The real danger of a big abstention could lead to a parliament where the disaffected determine the policy of the country. I don't think that is a healthy outcome."
Mr Blunkett, campaigning in south London, said: "I appeal to people to give power of leadership and strength to take decisive action rather than disable the Government."
Most senior Labour figures, including natural sympathisers on the left, reacted with disdain and anger to the defection. Barry Sheerman, who is defending Huddersfield, said: "He would have been a braver man if he had done it when he was a sitting MP.
"He is a man who felt always under Neil Kinnock, John Smith and Tony Blair he was overlooked for promotion and for a job. He had intellectual merit, but there was a lot of intellectual vanity and he spent too much time sitting in the tearoom nursing his grievances."
Peter Kilfoyle, a fierce critic of Mr Blair who is defending Liverpool Walton, said: "Brian Sedgemore is trying to cause maximum damage and embarrassment to the Prime Minister. But this isn't about the election of Tony Blair - it's about the election of a Labour government."
Diane Abbott, candidate for Hackney North and Stoke Newington, said: "Although I share some of his political differences with Tony Blair, Labour has a lot to be proud of and now is not the time to be leaving the party."
Jeremy Corbyn, the left-winger defending Islington North, said: "If you are serious about political change, it's only going to come through the Labour Party."
Malcolm Savidge, who is quitting as MP for Aberdeen North and has called for Tony Blair to step down, said: "I've never discussed the subject of joining another party with [Sedgemore]. It's not something I intend to do - I intend to fight for what I believe within the Labour Party."
Bob Marshall-Andrews, a left winger, said he felt "considerable sympathy" for Mr Sedgemore, a close friend.
"It saddened me but I must say that it doesn't altogether surprise me," said Mr Marshall-Andrews, who is defending Medway.
"There are undoubtedly a significant raft of Labour voters who have developed a very strong aversion to the Prime Minister."
Llew Smith, retiring as MP for Blaenau Gwent, said: "He was a fine parliamentarian and a grand socialist. It not only says something about Brian, but something about New Labour, that a person like Brian could be leaving the party. "I'm not surprised - others have been going. Thousands of people committed to similar ideas have left."
Tam Dalyell, who is retiring as an MP after 43 years, predicted that an attempt to remove Mr Blair would be mounted within months of the election.
He said: "I personally urge everybody to vote for a Labour government because, whether I'm naive or optimistic or not, I believe the Labour Party and Labour MPs can deal with the situation once the election is over."
One candidate said: "I'm telling people if you vote for Blair you will get Brown. Blair's days are numbered."
- INDEPENDENT
Defection rocks Labour's morale
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