A few weeks ago, it seemed impossible that a socialist would be in the running for the Labour leadership. The former miners’ leader Ian Lavery had ruled himself out and supported Andy Burnham; Lisa Nandy had resisted attempts to ‘draft’ her into the race. The debate was firmly anchored to the right, with Ed Miliband under attack for being ‘anti-business’ and focusing on the disenfranchised.

At a meeting of the Campaign for Labour Party Democracy, dismayed activists decided on one last push. Even if a leftwinger failed to get the required 35 nominations from MPs, they would put pressure on the other candidates. People were concerned that Burnham, having established himself as a committed NHS campaigner, was taking the left vote for granted. Jon Trickett, the shadow minister without portfolio, couldn’t be persuaded to step forward; Jeremy Corbyn’s campaign emerged from subsequent pressure to find someone else.

Seconds before the midday deadline on Monday, a late flurry of signatures saw Corbyn reach the threshold of 35. Jo Coburn on the BBC’s Daily Politicsdescribed them as ‘pity nominations’; there were reports that other candidates might 'lend' Corbyn some of their backers. But MPs weren’t allowed to transfer their nominations once they’d handed in their forms, though several apparently tried.

By the time Corbyn declared, a number of leftwingers had already pledged their support for Burnham, who looked likely to get the backing of the unions. Corbyn had only nine initial endorsements. But activists in the ‘Red Labour’ Facebook groups organised a social media campaign. There were 10,000 ‘likes’ for the official ‘Jeremy Corbyn for Leader’ Facebook page in 24 hours. It’s now on 23,000; Andy Burnham has 4500.

Lists of MPs who had yet to nominate were posted, and voters were encouraged to contact their constituency reps. When Mary Creagh dropped out of the race on Friday, her supporters were allowed to renominate. The next day they were the targets of a ‘Twitterstorm’ (#Jeremy4Leader) organised by Red Labour.

Centrist MPs such as Chi Onwurah, Clive Efford and Sarah Champion gave positive responses. Others weren’t so happy: the new Bermondsey MP, Neil Coyle, said messages were coming from Green supporters; Jonathan Reynolds said that the new process for electing the leader was about ensuring MPs’ approval, not ‘broadening the debate’.

He’s right. When Ed Miliband moved from an electoral college, itself a step forward from only MPs voting (pushed in the early 1980s by the late Vladimir Derer, the founder of the Campaign for Labour Party Democracy), to a ‘one member, one vote’ system, he raised the threshold for nominations as a sop to MPs who lost their block votes. But even some MPs are unhappy with the system. When David Lammy announced he was nominating Corbyn, he tweeted: ‘The next Labour leader should be chosen by members and supporters, not MPs.’

Now Corbyn has pulled through, his supporters must convince punters he is a credible leader, not a token anti-austerity voice. It’s unlikely he will win; in a preferential voting system, a polarising figure is unlikely to gain transfer votes. Still, the new system gives him a better chance than any previous left-wing contender. All votes count the same, including those of 'registered supporters' who can sign up for £3; plenty of people from the non-Labour activist left will do this now they can vote for Corbyn.

A straw poll on the Mirror website puts him miles ahead at 57 per cent, and the grassroots site LabourList puts him on 47 per cent. Neither is a scientific sample, and they were probably influenced by social media excitement, but the LabourList surveys are a good barometer of rank and file feeling.

When Ed Miliband beat his brother in 2010, the Times columnist Danny Finkelstein said it was ‘Vladimir Derer’s revenge’ – the first time a leader had won without the backing of MPs. A strong showing for Corbyn this time round could – unwittingly – be Miliband’s.