After Ed Miliband resigned, the acting Labour leader Harriet Harman said the system for electing his replacement would ‘let the public in’ to the debate:

This is the first time a political party in this country has opened up its leadership contest in this way and I think there will be a real appetite for it out there... We should not be afraid of differences. We should thrash them out.

Under the new rules, each vote has equal weight, including those of ‘registered supporters’ who can sign up for £3 – making the election more like a US primary. The changes were overseen by Ray Collins, the party’s general secretary from 2008 to 2011. Trade unionists who were once sent ballots automatically now have to opt in. When the Collins review came to a vote at a special conference in March last year, the only opponents were grassroots activists on the left, who argued that it undermined the principle of collective affiliation. But it’s the candidate of the left, Jeremy Corbyn, who looks set to benefit most. Last week the first scientific opinion poll put him ahead, beating Andy Burnham by 53 per cent to 47 per cent after second and third preferences were reallocated.

At the weekend, the Sunday Times reported that the huge numbers of new members and supporters signing up to Labour were down to a ‘hard left infiltration’. The paper said the Communist Party of Great Britain (Provisional Central Committee) ‘has called on supporters to join and back Corbyn as part of its revolutionary “strategy”’. It didn’t mention that the party is a splinter group of a few dozen members.

John Mann, the Labour MP for Bassetlaw, said the contest was ‘out of control’ and called for Harman to suspend it, pending checks on new joiners. (Harman has since said that they are ‘policing the integrity of this process’.) ‘It is becoming a farce with longstanding members,’ Mann said, ‘in danger of getting trumped by people who have opposed the Labour party and want to break it up, expressly want to break it up – some of it is the Militant Tendency types coming back in.’ Yet Mann was a champion of ‘letting the public in’ long before it was received wisdom in Labour, making the case in Progress magazine in 2013. In the 2010 leadership contest, Bassetlaw was the only constituency to determine its nomination – and Mann’s vote – by a primary of Labour supporters.

Still, it’s no surprise that Corbyn’s opponents on the right of the party have resorted to questioning the process. They did the same thing with the row over Unite's influence on the Falkirk parliamentary selection (which prompted the Collins review in the first place). But the political attacks on the leftwinger have mostly backfired. Earlier last week, Mann accused Corbyn of inaction over a child abuse scandal in his constituency in the 1980s. A spokesman for Corbyn said it was ‘a new low’; hundreds of activists on social media agreed.

Tony Blair said Corbyn’s supporters needed ‘a heart transplant’. Corbyn’s campaign said they received more than 1000 emails from activists offering their support on the day of Blair’s intervention – far more than any previous day. At an early hustings at the GMB union’s annual congress in Dublin, the other candidates barely engaged with Corbyn’s arguments, presumably thinking he would fail to get the nominations. Luke Akehurst, the secretary of the ‘old-fashioned’ (non-Blairite) rightwing faction Labour First, said Corbyn should be on the ballot to prove the weakness of the left. ‘I want his ideas scrutinised and defeated in a democratic ballot so we can prove that they are not the direction the vast majority of members want to go in,’ he wrote. Now the candidates have learned to ignore or underestimate Corbyn at their peril. Liz Kendall said last week that Corbyn was gaining ground because members had ‘been through a huge trauma’ after the election defeat.

Party grandees’ shock at Corbyn’s success points to a gulf in political opinion between the parliamentary party and the wider membership. In the 1970s and early 1980s, constituency delegates at Labour conference would lay into MPs for betraying the principles they espoused when selected as candidates. There was a decline in both membership and grassroots activity in the Blair years. Party members became more willing, as the former national executive member Liz Davies put it, to play 'a walk-on part'. But now Corbyn is leading in constituency nominations, which the new registered supporters have no influence over. Unlike in 2010, many MPs have been unable to persuade their local parties to endorse their preferred candidates. The frontbenchers Gloria de Piero, who nominated Kendall, and Jonathan Ashworth, who nominated Cooper, both looked on as their branches nominated Corbyn.