Image copyright Getty Images

Labour leader Jeremy Corbyn's supporters are focused on the campaign leading up to the election on 8 June. But what about those looking beyond that?

Anyone in the media, or indeed the Labour Party, who suggests Jeremy Corbyn might lose the election is usually treated to a Twitter storm from his staunch supporters.

Yet the leader of the union which not only generously funds the Labour Party - but has also contributed significant sums to Jeremy Corbyn's two leadership campaigns - has predicted defeat.

Media playback is unsupported on your device Media caption Len McCluskey spoke of his "optimism" on Wednesday

Unite's Len McCluskey later tried a classic obfuscation technique, claiming now he has seen the Labour manifesto in full, he is full of optimism and looking confidently forward to the campaign.

The awkwardness about that is not only that Mr McCluskey was in the meeting that signed off the manifesto before its publication. But also that the interview where he predicted defeat was carried out an hour after the manifesto was finally published in any case.

It seems likely that his thoughts during the Politico interview were focused on 9 June, not 8 June.

Because the question being discussed privately amongst those close to Jeremy Corbyn is whether he should stay if Labour loses - and for how long.

His opponents inside Labour want him gone. They had hoped if Len McCluskey's recent challenger for the position of Unite's general secretary - Gerard Coyne - had won, it might have been possible to get a triumvirate of union leaders to call for him to go, the day after the election.

Any successor would then need 15% of MPs and MEPs to nominate them to get on the ballot - almost certainly excluding someone as left wing as Jeremy Corbyn from the process.

The door would then be open to other candidates who Labour MPs think are likely to run such as Yvette Cooper and Keir Starmer.

But of course that didn't happen and there are signs that Mr Corbyn won't go quietly.

Improving on Miliband's share?

So while Labour candidates are of course mainly focused on trying to keep their seats or win more, among both Mr Corbyn's critics and his supporters, a lot of energy is being devoted to planning for the aftermath of a defeat, if the polls prove accurate.

Those around Mr Corbyn are stressing the real test in the election is vote share and not the number of seats gained.

Their argument is that if he improves on Ed Miliband's share of the vote - i.e. 31 rather than 30.5 per cent - he should be allowed to continue his work of transforming the Labour Party.

But his opponents and party officials say the gap between the two main parties is a better measure - with the expectation that the Conservatives will be further ahead than the 6.5% gap in 2015.

Image copyright Getty Images Image caption Len McCluskey's allies say he has benchmarks for the Labour leader

So where does Len McCluskey stand in this debate?

Privately, one of his key allies says he does have performance benchmarks for Jeremy Corbyn.

We have now seen him mention publicly a target of retaining 200 seats, despite his attempts today to distance himself from his own comments.

But we have also been told privately that vote share matters too - if Labour finishes on 29% of the vote or below, he is likely to tell the Labour leader to go.

But not immediately.

He would favour keeping Jeremy Corbyn in place until the autumn party conference, where there is at least a chance of changing the leadership election rules in favour of the left by requiring fewer MP nominations to get on the ballot.

But he wouldn't favour Jeremy Corbyn himself going on and on simply to avoid the risk of a more right wing successor.

Who calls the taxi?

Now this is not to say Len McCluskey is speaking on behalf of everyone on Labour's left.

A shadow cabinet ally of Jeremy Corbyn's described his remarks about a defeat as a "disgrace".

And sources suggest that he is a less powerful figure than he once was. The perception that he alone could guarantee the leader's position or decide his fate has faded in some powerful quarters.

Media playback is unsupported on your device Media caption Jeremy Corbyn sets out Labour's manifesto

Mr McCluskey may have won his own leadership election, but he didn't cruise to victory - he won narrowly on a low turnout.

And by some counts, it is Dave Prentis, the leader of the health union, Unison, who is the leader of the biggest union now.

He made plain his priorities in a short but pointed tweet after McCluskey's comments: "Success = a Labour government. That's what care workers, nurses and teaching assistants need. Let's get out there."

It's thought that Unison and the GMB, the third of the big three, would also urge Mr Corbyn to walk.

But while Mr McCluskey may no longer have the power to keep him in place, his voice matters hugely. One well placed insider said: "It's only Len that can tap Corbyn on the shoulder and tell him it's all over."

Another said: "It will be Len who calls the taxi."

Polls and pundits' predictions

But strangely, even if some of Mr Corbyn's most bitter critics are open to the idea of some kind of staged departure in the event of a heavy defeat, one senior figure says he has to be allowed to have a "dignified exit".

The plan is that he would agree to go quietly at the party's conference in the event of defeat.

Another told us: "This has to be Corbyn's election - he has to own the defeat."

So any rapid putsch from his internal enemies is unlikely: "There's a tactical decision not to be blamed - we have to grit our teeth and watch the car crash happen, the party will have a choice - thoughtful reflection or plunge straight into another civil war."

It's understood that potential candidates for the job are being urged privately to bide their time in the immediate aftermath of the election in the case of a bad defeat, rather than rush straight out of the traps and give Mr Corbyn's supporters the impression that most of the PLP have been plotting against him all along.

The morning after the election could of course feel totally different. There is a long way to go in this race.

Labour could well wake up to a result far better than expected. Mr Corbyn's legions of supporters could mobilise in the ways he hopes and surprise the pundits once more.

But, if the polls are broadly right, the party could face questions about its actual survival. The election battle could turn in to a fight for Labour's future soon after polling day.