Labour risks a split because of the provocative language used by supporters of far-left leadership candidate Jeremy Corbyn, rival Andy Burnham has warned.
Mr Burnham's warning came after union leader Dave Ward described Blairites in the party as a "virus" to which Mr Corbyn was the antidote.
The shadow health secretary said the comments should sound "alarm bells" to the party of the danger of a return to the early 1980s, when a bitter rift between left-wingers and centrists led to the creation of the breakaway Social Democratic Party.
Mr Burnham's warning came as a poll suggested he was the preferred Labour leader among people who voted for the party in the general election, as well as the favourite of the general population, despite indications that he is trailing Mr Corbyn by a large margin among those who actually have a ballot to cast.
When constituency party nominations closed at midnight, Mr Corbyn led the field with 152 to Mr Burnham's 111, Yvette Cooper's 106 and Liz Kendall's 18. Constituency supporting nominations have no weight in the ballot which will produce a leader on September 12, but offer an insight into the preferences of the most active party members.
In the deputy leadership race, Tom Watson secured an overwhelming lead in constituency nominations with 174, more than twice second-placed Stella Creasy's 77, and well ahead of Caroline Flint on 64, Angela Eagle on 32 and Ben Bradshaw on 20.
Among 1,001 members of the public questioned by market research company Research Now, Mr Burnham was picked as best Labour leader by 30% of those who expressed an opinion, ahead of 24% for Mr Corbyn and Ms Cooper and 21% for Ms Kendall.
When the same question was put to those who backed Labour in May - many of whom will not be registered to vote in the leadership election - Mr Burnham extended his lead on 36% to Mr Corbyn's 28%, with Ms Cooper (20%) and Ms Kendall (16%) trailing.
A spokesman for the Burnham campaign said: "Andy is the clearly the candidate to win back lost voters to Labour and unite the party at the same time."
But one MP supporting Ms Cooper poured cold water on the findings, saying: "Silly season continues.... Time for leadership candidates to focus on the big challenges facing our country and our party rather than fantasy figures from 'research companies' with little or no experience in UK politics."
Communication Workers Union deputy general secretary Mr Ward stood by his comment, saying: "I want to see a party that stands up for workers again, that stands up for the disadvantaged in society, and puts those principles first without compromising those principles just in pursuit of an election victory."
But Mr Burnham told The Guardian the remark was "very provocative and frankly unhelpful".
He warned: "There is a factionalism that is bubbling under here which, to his credit, Jeremy has not (endorsed) ... There is a risk, though, of a split if people talk in that provocative way.
"There are dangers here, there are some echoes of the early 80s. Those should ring loud warnings, alarm bells ... Labour needs to reflect on this, and not go down that path."
Trying to explain Mr Corbyn's apparent appeal, Mr Burnham said: "The party is hungry for something different. It wants a bigger thing. It has been fed this diet of thin, meagre gruel of gimmicky policies. It is hungry. That is why it laps it up."
But he insisted that his own election manifesto, due to be launched next week, "will set out the most radical Labour vision since the post-war government".
Meanwhile, former shadow chancellor Ed Balls stressed the importance to Labour of not getting a reputation as "anti-business".
While making no direct comment on the leadership contest - other than to confirm he will vote for his wife Ms Cooper - Mr Balls told the Daily Telegraph: "It was no secret that I thought from the very beginning that Ed (Miliband) needed to position himself better with business. I said to him very early on that we should come out for the new runway at Heathrow because it would be a very good signal of long-termism."
And he said Mr Miliband's flagship energy prize freeze had given the impression that "energy companies are the bad guys".
"If you allow yourself to be anti-business, or anti-bank or anti-energy company, people think, 'Hang on a second, you have to work with these people if you want to govern'," said Mr Balls.