Owen Smith has accused Jeremy Corbyn and his supporters of "bullying and intimidation" and warned there were "five days left to save the Labour Party".
In a speech in London, the challenger and rank outsider claimed Corbyn backers in Momentum were using the same tactics as the Militant Tendency in the 1980s and "the very future" of Labour was at stake.
Mr Smith's outspoken attack on Mr Corbyn's left-wing allies comes in his final speech of a bitter leadership campaign and follows his bruising clash with the Labour leader in the final TV debate.
It also comes after the disclosure of moves by left-wing activists to de-select the veteran Liverpool MP Louise Ellman and a leaked document that named and shamed Labour MPs "disloyal" to Mr Corbyn.
But it has almost certainly come too late to influence the outcome of the contest.
Bookmakers, pollsters and most Labour MPs believe Mr Corbyn is on course for his second decisive victory in a year.
Mr Smith said: "With five days left in this race, there are five days left to save the Labour Party. Five days to save it from a leadership that's taking the party away from the concerns of the British people, away from credibility, away from unity.
"Five days to get the Labour Party into a position where it can face outwards and talk about the problems the country faces and the solutions we offer - not face inwards with talk about deselections and Red Tories and splits."
And in a blistering attack on the left, which many Labour MPs disappointed by Mr Smith's campaign claimed he should have made much earlier, he said: "The very future of the Party is at stake.
"Just as we had to fight Militant in the 1980s, we have to fight to save the Party we love today. We know what's at stake, because they've started showing their hand.
"Momentum in Liverpool - some of them exactly the same people as were in Militant all those years ago - organising to deselect a Labour MP. Threats to MPs all over the country.
"A 'deselection list' circulated by Jeremy's campaign, attacking Labour MPs. Disowned by the campaign. Apologised for by the campaign. And then defended, on stage at this week's hustings, by Jeremy himself.
"It's part of a culture of bullying and intimidation that says to Labour MPs and Labour members that they have to shut up and get in line or get out. Exactly the same tactics used by Militant in the 1980s.
"What are Momentum doing organising a conference alongside Labour Party conference? What are they doing organising to push out long-standing Labour representatives? What are they doing turning a blind eye to abuse and booing and hatred?
"As John McDonnell said of himself, it uses Labour membership as 'a tactic… If it's no longer useful, move on'.
"I don't think Labour membership is a tactic. I don't think the Party is a vehicle. I'm not relaxed about seeing the Party split 'if that's what it takes'. Our movement is more than that, and I love it too much to see it die."
Mr Smith added: "We are the party of Jeremy Corbyn and Tony Blair - not the Party of Jeremy Corbyn or Tony Blair. And those of us who don't think of ourselves as either Blairites or Corbynites - which is most of us - are welcome in this Party too.
"The whole Party stood together against apartheid, marched in solidarity with striking miners, fought against poverty and inequality here at home, asserted our country's moral duty to support the world's poorest people. Those weren't fringe positions taken on by lone prophets: they were held across the Labour movement.
"If we lie to ourselves about what this Party is and has always been, if we choose to pretend that what we have all always stood for is something we have only just discovered, then we do an injustice to all of our predecessors in the Labour movement.
"And if we take pride in our narrowness as a movement, instead of celebrating the breadth of our Labour tradition and everything that we have achieved together, then we make it less likely that we can appeal to the public, and less likely that we can win. If we hate our own history, why should anyone else love it?"