Andy Burnham has hit out at Labour leadership rival Jeremy Corbyn's anti-austerity promises, claiming that "credibility" on the economy is essential if the party is to win the next election.
The shadow health secretary and the veteran left-winger, who have emerged as the two leading contenders in the contest, were fighting a battle of "big visions" about the future of the party and country, Mr Burnham said.
Mr Burnham's comments came as Mr Corbyn set out his plans for northern England and praised the Labour administration of the late 1970s - which was ultimately defeated by Margaret Thatcher.
Although Mr Burnham is the bookmakers' favourite to claim victory on September 12, Mr Corbyn has received the symbolically important backing of more constituency Labour parties and influential trade unions.
Mr Burnham appealed to Labour supporters not to take the party "backwards" as he launched a direct attack on Mr Corbyn's economic plans, which include the possibility of the Bank of England effectively printing money to finance public investment.
He told the Evening Standard: "The bottom line is you don't get out of first base at an election unless you have credibility on the economy and the public finances.
"I don't believe that we did not win the election because we were not left-wing enough."
Mr Corbyn's surge in popularity has seen crowds pack out his meetings, and Mr Burnham acknowledged "the race has come to life".
"It's becoming a contest now of big visions," he said.
In his interview Mr Burnham set out a range of housing and transport reforms which will be in his forthcoming manifesto, including measures to guarantee people a home to rent or own, controls to stop rents spiralling out of control, and increased security of tenure.
He firmly backed a third runway at Heathrow but hinted that the HS2 rail line could be a lower priority than measures to boost east-west links.
In Leeds, Mr Corbyn set out his Northern Future policy document, which called for a rebalancing of investment.
"The North was the industrial heartland of Britain," it said. "It was weakened as an act of deliberate policy, and requires improvements in transport, educational access, and concentrated industrial funding to rebuild."
Mr Corbyn claimed the country was still "paying the price" for the Thatcher administration's policies as he praised the work done by defeated Labour prime minister James Callaghan for investing in the high-tech industries of the time.
He said: "I'm accused of being a throwback to the 1980s, but I'll go back one decade further and just say there are lessons to be learned from what that government was trying to do.
"To recognise the changing industrial face of the whole world also recognises the huge skill levels we have and the traditions of industry we have in this country that were allowed to be wasted on the altar of monetarism and the development of a financial services economy, rather than a manufacturing economy."
Dismissing the personal criticism aimed at him, and rumours in Westminster that fellow Labour MPs could seek to oust him if he won, Mr Corbyn added: "It is the name-calling, the depoliticisation of serious political debate that drives people away.
"Our campaign is not getting involved. We aren't doing rebuttals. We're not interested."
Meanwhile, fellow leadership hopeful Yvette Cooper enjoyed a boost to her campaign after former home secretary Alan Johnson endorsed her.
"I believe that Cooper has the intellect, the experience and the inner steel to succeed in this most difficult of roles," he wrote on the Guardian website.
"In my view only Cooper can unite the party to win again."
In a sign of Labour's difficulties, community organiser Arnie Graf, who was brought over from the US to help under Ed Miliband's leadership, revealed that for one event, local activists had not been able to find a worker on the minimum wage to appear with the then leader.
In an article on the LabourList website Mr Graf said he was able to locate someone to take part in the event but added: "How could it be that the Labour Party, supposedly the party of working people, was not in relationship with a single minimum wage worker? It was stunning!"