Labour has "rediscovered losing", former Prime Minister Tony Blair warned as a poll put left-winger Jeremy Corbyn ahead in the leadership contest.
Mr Blair said Labour could win again - but not from a "traditional leftist platform" and said it had to "move on".
Mr Corbyn said it was "a bit premature" to talk about him winning.
The YouGov poll for the Times suggests that in the final round of voting, the Islington North MP would finish six points ahead of Andy Burnham.
It shows Mr Corbyn as the first preference for 43% of party supporters, ahead of Mr Burnham on 26%, Yvette Cooper on 20% and Liz Kendall on 11%.
Taking second preferences into account the poll, of 1,054 people eligible to vote in the contest and carried out between Friday and Tuesday, pointed to a 6% Corbyn victory.
Yvette Cooper and Liz Kendall are the other candidates for the leadership.
Addressing the Progress think tank, Mr Blair said the "debilitating feature" of the leadership contest was that it was being presented as a choice "between heart and head", adding that people who say their heart is with Mr Corbyn should "get a transplant".
Even if Mr Corbyn could win power, something Mr Blair thought would be unlikely, he said it would not be the right choice for the country.
"It would not take the country forward, it would take the country back," he said.
Since he became Labour leader 21 years ago, the party had "discovered winning successively" then "rediscovered losing successively", he said, adding: "Personally I prefer winning."
Labour should not "despair", he said, adding that the Conservatives were "vulnerable" and were "going to get cocky" in government.
He recalled the 1979 election, when Labour adopted "the theory that the electorate is stupid" and responded to a Conservative victory by moving to the left, going on to lose three more elections.
'Back to the 1980s'
May's election was "out of the playbook of the 1980s" with the Conservatives seen as the party of economic competence and Labour of compassion, he said, adding that the party should avoid repeating past defeats by occupying the centre ground of politics.
Describing his "disagreement" with former Labour leader Ed Miliband - who he paid tribute to for his courage, he said: "He thought the centre ground had shifted left, I do not think the centre ground shifts in that way."
Mr Blair also attacked the SNP, saying nationalism was "caveman politics".
Mr Corbyn said he did not want to discuss opinion polls but said his campaign was going "extremely well".
"A lot of people are supporting us, particularly young people who want to see a Labour Party that is very different to the Labour Party they have had in the past," he added.
One of Mr Corbyn's supporters, former London mayor Ken Livingstone, said he had "got the economic strategy right" and that it was "waffle" to suggest it would divide the party if he won.
But Yvette Cooper said the party would not win by either moving sharply to the left or right.
"We do have to stand up for the ideas we believe in but we have to make sure that we can deliver," she said. "Otherwise it is people who depend on Labour who are going to be let down by this. That is why it is so important and serious that we are credible".
Shadow Education Secretary Tristram Hunt, who is backing Ms Kendall for the leadership, said there was a "big, philosophical struggle" going in within the party and Labour's "progressive, modernising" wing had not made a good enough case on issues like public spending and welfare reform.
Mr Corbyn's supporters, he said, wanted to "rack up" more votes in Labour areas rather than Conservative ones, comparing the left-winger's popularity to that of anti-austerity parties in Greece and Spain.
Labour announces its new leader at a special conference on 12 September.
Analysis by Anthony Reuben, BBC News head of statistics
You would expect people who are eligible to vote in the Labour leadership contest to be unusually interested in politics.
But the full details of the poll show that of the 1,056 people polled, 80% said they would definitely be voting in the leadership election while another 15% said they would probably be voting.
I know that the rules for the leadership election have been changed, but it still seems a very high turnout.
Compare it with the 2010 leadership election, when about 72% of constituency Labour Party members voted.
Former Tony Blair adviser John McTernan told BBC Newsnight two of the other candidates should withdraw in favour of an "anyone but Corbyn" candidate.
He also said Labour MPs who lent their support to the Islington MP - who struggled to get enough nominations to make it onto the ballot - were "morons".
Asked whether she regarded herself as a "moron", former foreign secretary Margaret Beckett - among those to nominate Mr Corbyn - acknowledged she was "one of them".
She told BBC Radio 4's World at One that she never intended to vote for Mr Corbyn but thought, at the time, that it was "legitimate" to nominate him to ensure as wide a field of candidates as possible.
Former deputy leader Lord Hattersley urged Labour supporters "not to lose their nerve", saying Mr Corbyn would not be elected leader and the current speculation about him winning was "no more than a 24-hour sensation".