Jeremy Corbyn has responded to his critics in the Labour leadership contest by calling for an end to "silly remarks".
The left-wing MP said politics should be "comradely and friendly".
Former Cabinet minister Alan Milburn followed Tony Blair in warning against a victory for Mr Corbyn, saying Labour had a "death wish" if it shifted left.
Another candidate, Liz Kendall, said a win for Mr Corbyn - who topped a recent poll - would be a "disaster".
Labour party donor John Mills told the Guardian the election of Mr Corbyn could trigger an "SDP-style split" and the withdrawal of support from wealthy donors.
The Labour row comes after a YouGov poll for the Times that put left-winger Mr Corbyn ahead in the race and Ms Kendall fourth.
Mr Blair has warned the party would not win from a left-wing position, and one of his former advisers said MPs who helped Mr Corbyn onto the ballot paper but did not support him were "morons".
'To the end'
Asked about the criticism, Mr Corbyn told the BBC: "I think politics should be conducted on a comradely and friendly basis, and if people disagree with each other then say what they disagree on, and let's keep these silly remarks to themselves."
Ms Kendall dismissed calls for her to withdraw from the contest to ensure Mr Corbyn is beaten.
She told the BBC: "I'll be fighting for what I believe in until the very end."
She said a victory for Mr Corbyn would be a "disaster", saying that turning back to the politics of the 1980s and losing elections "does nothing to help the people" the party wants to help.
Labour leadership contest
- Who are the candidates? Andy Burnham, Yvette Cooper, Jeremy Corbyn, Liz Kendall
- Dates: Ballot papers will be sent out on 14 August; voting can take place by post or online. They must be returned by 10 September. The result is on 12 September.
- Who can vote? All party members, registered supporters and affiliated supporters - including those joining via a union - get a vote
- What is the voting system? The Alternative Vote system is being used so voters are asked to rank candidates in order of preference
- How does it work? If no candidate gets 50% of all votes cast, the candidate in fourth place is eliminated. Their second preference votes are then redistributed among the remaining three. If still no-one has reached 50% of the votes the third place candidate is eliminated with their second preferences (or third in the case of votes transferred from the fourth place candidates) redistributed. It is then a straight head-to-head between the last two candidates with the one having the most votes winning.
Mr Milburn told the BBC the party would only win the next election if it occupied the centre ground.
He added: "The alternative is that Labour lurches to the left. If it does that, frankly, it has a death wish."
Mr Mills, a donor to the Kendall campaign, told the Guardian: "If Corbyn won, I suspect what would happen is that there would be some sort of split.
"Then you would have an SDP-type party" - a reference to the Social Democratic Party formed in 1981 by Labour party defectors.
Mr Mills said he suspected "some of the major donors" would be "less likely to give".
The YouGov poll suggested Mr Corbyn could receive 43% of first preference votes, ahead of Andy Burnham on 26%, Yvette Cooper on 20% and Ms Kendall on 11%.
In a subsequent poll, for Ipsos Mori, 27% of the general public said Mr Burnham would be a good prime minister, compared to 22% for Ms Cooper, 17% for Mr Corbyn and 16% for Ms Kendall.
Mr Burnham also led among Labour supporters.
On Tuesday, former prime minister Mr Blair warned Labour would not win again from a "traditional leftist platform".
He also said people who had Mr Corbyn's politics at heart should "get a transplant".
This prompted a response from former Labour deputy prime minister Lord Prescott, who said Mr Blair was wrong to "abuse" supporters of Mr Corbyn and told the party to "calm down".