Labour Party MP David Shearer has come out swinging against former leader David Cunliffe, saying he will end up undermining whoever takes over the leadership.
Mr Shearer has confirmed he will not be seeking the leadership of the party but is disappointed that Mr Cunliffe withdrew from the contest.
With nominations closing later today, the declared contenders are Andrew Little, David Parker and Grant Robertson.
Mr Cunliffe pulled out of the leadership race yesterday and said he would support whoever won the contest - although he immediately endorsed Mr Little.
Mr Shearer said he would have liked Mr Cunliffe to stand as if he did not win, it might have curbed him - and particularly his supporters - from undermining whoever became the leader.
"There are people who David surrounded himself with, some of whom are working in his former office, who actively undermined me [as leader] and undermined [former leader] Phil Goff.
"These are people who sit behind darkened screens and blog and undermine people.
"What I'm saying is let's have a clean start, let's move on behind a new leader, without any murmur or dissent or whatever. That is absolutely the most important thing."
Mr Shearer said the party needed to be less focused on special interest groups and to broaden its appeal to win the country's support. As well, attacks on individuals needed to stop and he would therefore have preferred Mr Cunliffe to stand, so there could be a clean break if he lost.
"At the moment it's a sort of a Twilight Zone before something else happens," he said.
But Mr Little believed Mr Cunliffe had made the right choice in deciding not to stand.
"I think it's good for the party and it's good for David. While I welcome any support I can get, I don't underestimate the effort I have to go to to win the confidence of the party, the affiliates and indeed the caucus if I am to win this contest."
Mr Little said he was confident that whoever wins will have the full support of the caucus.
"When we get through this period everybody understands it is going to be time for knuckling down, single focus and getting on with the job and David Shearer will be as good as anybody at doing that."
Mr Parker said he believed Mr Cunliffe could buckle down under a new leader - but that he would not have him as his deputy, should he win.
"It's not because I have any bad feelings towards him, it's just that I don't think it would work."
Mr Parker said he was not daunted by the fact that Mr Little might pick up more support with Mr Cunliffe's endorsement.
Mr Robertson said he would have liked an endorsement from Mr Cunliffe but that the latter was entitled to state his preference - something others would do in time, too.
He said the party's caucus has no choice but to be unified in the outcome of the leadership contest.
"One of the things that we've got to be sure of is that we only have one leader over the next three years.
"Once we make this decision the person who gets elected needs to know that, needs to have the confidence that we can go forward."
Labour's caucus meets this morning and will decide whether Mr Parker should remain the acting leader during the leadership contest.
Nominations close for the leadership at 5pm, and the first husting meeting is in Wellington next Wednesday.