Labour faces "division and destruction" if Jeremy Corbyn recaptures the Labour leadership next week (24 September), Owen Smith has warned.
The challenger also branded shadow chancellor John McDonnell a Marxist, claiming Corbyn and pressure group Momentum were attempting to take Labour to the "hard left".
"I am a Keynesian, not a Marxist like our current Shadow Chancellor, and prudent borrowing and government partnership with business to grow our economy is the answer, not a longed-for crisis in capitalism that destroys pay-checks and pensions en route to some fabled revolution," he said.
"Most importantly, I am a democratic socialist — it says so on my party card — and I believe our duty is to win power in the country, at the ballot box, through making our case in parliament. To win the trust and seek to serve the British people.
"Because it is only through building a coalition of support to command respect and win power that we can ever enact our values and make Britain the secure, prosperous, fairer and more equal country we dream of.
"And honestly, right now, with Jeremy Corbyn as our leader, I see no chance of that. In straight-talking terms, we are heading for division and destruction."
But despite Smith's plea, Corbyn, who secured almost 60% of the vote last year, is expected to win the latest leadership election. Bookmaker William Hill has the left-winger as 1/25 favourite (96% chance), while Ladbrokes has Corbyn at 1/33 (97%).
The Labour leader also has the most constituency party nominations, with a poll from YouGov for The Times, of more than 1,200 Labour members between 25 and 29 August, putting him on a 24 point lead over Smith (62% versus 38%).
But Smith claimed he was still in the running, citing the endorsement of the GMB Union after a poll of its members. "Don't write me off until the final whistle, is all that I ask," he told IBTimes UK.
The former shadow work and pensions secretary has attacked Corbyn and his allies for creating divisions within the Labour Party.
IBTimes UK asked Smith if Corbyn could make a credible unifying gesture if he retains the leadership.
"I don' think there is one big gesture," Smith said. "I think he will have to work incredibly hard to rebuild the bridges that have been burnt over the last year, to try and illustrate to the Labour Party in parliament that he is serious about taking on the Tories.
"More importantly than that, he needs to start proving to the country that he is someone that can be a prime minister. Because the polls on Jeremy's leadership have been worse than they've ever been for any leader of the Labour Party in living memory."
The comments come just a day after McDonnell, the chair of Corbyn's re-election campaign, apologised for the leak of a 13-strong list of "hostile" Labour MPs to the press.