Image caption Jeremy Corbyn and Owen Smith have both set out their stalls to become the next Labour leader.

Can this summer's Labour leadership contest unite the party's warring factions?

This week's statement by Iain McNicol, general secretary of the Labour Party, was blunt: "There is no place for abuse of any kind in the party," he declared. "There is simply too much of it taking place and it needs to stop."

Politics involves debate and sometimes it gets heated; but when a party has to restrain the discussion within its own ranks, something has gone seriously wrong.

Labour is holding a leadership election this summer, but has had to suspend its most basic forums for debate - the regular meetings of its constituency parties. That's how bad things have got between those who support Jeremy Corbyn and those who do not.

I heard an example of this from Nora Mulreedy, a member from north London. When she complained about postings in a closed Facebook group in which a fellow member had referred to "killing" disloyal MPs, she found herself the subject of complaint for raising it in public.

Mr McNicol argues that condemning abuse is not enough, and he urged members to report such behaviour to Labour's "validation" department. I assume these are the same people currently scrutinising those wanting to vote in this leadership election who applied during last week's 48-hour window for registered supporters.

According to those involved, there are just 15 people scrutinising around 183,500 would-be voters. Software "cobbled together in-house" is checking social media posts for any evidence that applicants don't share Labour's values.

They're "looking at bile", a source inside the party says, and expect a number in the low thousands to be rejected for that reason.

On the surface, Labour is in rude health. Its size has more than doubled in a year, to more than 515,000, something that these days is pretty rare.

Many in its establishment, though, are nervous of these new members; older hands fear a re-run of the early 1980s, when what was called the "Militant Tendency" was accused of infiltration.

Image caption The telephone canvassing event at the offices of the Unite union was organised by the Momentum group which supports Jeremy Corbyn

At a telephone canvassing event at the offices of Unite, one of the big unions which affiliate to Labour, one woman told me her attempts to join the party had been rejected; for nearly a year, she'd tried without success to get an explanation.

She told me she'd last been in the party in the early 1980s, and hadn't belonged to any other party since. She did admit to having been a member of Militant at the time, "but that was thirty years ago," she said. "I'm over sixty now, I work with children, I'm not going to be lobbing bricks through anyone's window."

A few years ago, she might well have been readmitted. After all, some ex-Militants even ended up as MPs (one I know of was a pretty successful Business Minister). On the other side, quite a lot of those who defected to the SDP also rejoined without fuss.

Now, though, suspicion and mutual distrust suggest neither side is willing to believe the declared motives of the other. Abuse is a symptom of that, magnified by social media which seems to have the same effect on some people as getting behind the wheel does on others. Twitter rage, like road rage, has its casualties.

Image caption Heidi Alexander, Liz Kendall and Lillian Greenwood are among those MPs who have written to Jeremy Corbyn to complain about abuse from his supporters

Professor Tim Bale and colleagues at Queen Mary, University of London surveyed new members. Of those who've joined since 2015, only around 30 per cent of them have delivered leaflets and half that number have done canvassing.

"Far more of them actually are prepared to re-tweet and to share things on Facebook. These are people who are in politics electronically, as it were; they're clicktavists, not activists."

Things will be particularly intense online and everywhere else between now and 8 August, the final deadline for leadership votes.

It's too late to become a party member or registered supporter; but members of affiliated organisations, including unions and the quaintly named Woodcraft Folk among others, can apply for a vote until that date.

Nora, who I mentioned earlier, is part of Saving Labour, the online campaign seeking to counter the Corbyn surge. Her opposition to Mr Corbyn remaining leader is because she believes he lessens the chances of a general election victory. She's irritated that people like her are dismissed as "Blairite", a word that's used as a term of abuse.

"If wanting a Labour Government that can fight against poverty in a meaningful way, changes laws, gets people more money in their pocket makes me a Blairite, fine. I'll take that label and wear it proudly," she says.

Still, there are signs that the ten months of Mr Corbyn's leadership have diminished some of the euphoria.

The telephone canvassing event I mentioned was organised by Momentum, a group which helped him to win. During the telephone conversations, I noticed one of the questions being asked was a respondent's "reason for dropping support for JC". Among the potential boxes to tick in response was "bad party leader".

Image caption Momentum is a grassroots organisation which was founded in the wake of Jeremy Corbyn becoming Labour leader

One of those I met there was Noel, a smartly dressed elderly man who'd joined Labour on leaving the RAF in 1969. He's frustrated at the growing gulf between Corbyn supporters like him and most of the party's MPs.

"The Labour Party in parliament is a flower without any roots," he says. "The party in the country are roots without any flowers."

Jeremy Corbyn's critics doubtless would say the roots are fine, but too many weeds have now attached themselves.

Yet Mr Corbyn still enjoys one big advantage - the passion of his supporters. By contrast, at the end of my interview with Nora and two of her fellow supporters of Saving Labour, my producer noticed that they hadn't mentioned Owen Smith, the leadership challenger, at all.

Shaun Ley presents The World This Weekend during the summer, at 1300 BST on Sundays, and afterwards available on the Radio 4 website.