Image caption Current Unite General Secretary Len McCluskey faces a challenge from Gerard Coyne, as well as Ian Allinson

Asked if Liverpool Football Club's next match was a matter of life and death, the late-lamented manager Bill Shankly replied: "No, it's much more important than that".

Now, a present-day son of that city is facing an epic showdown which may feel to him even more important than football. And it could turn out to be a proxy battle for the very future of the Labour Party itself.

Len McCluskey is not only the General Secretary of Britain's biggest union, Unite. He also happens to be one of Jeremy Corbyn's most formidable protectors and defenders.

He has been credited with seeing-off at least one threatened coup against the Labour leader and is generally considered one of the union movement's heaviest hitters. But it was his muscular embrace of Mr Corbyn which made him headline news after that sensational party leadership contest in 2015.

Mr McCluskey wasn't originally due to stand for re-election until next year. But he has brought it forward in the hope, say his critics, of beginning his drive for re-election while catching potential challengers on the hop.

That has not stopped Unite's West Midlands Regional Secretary Gerard Coyne from joining the contest with his own election manifesto published this week.

Image caption Len McCluskey has been a vocal supporter of Labour leader Jeremy Corbyn

Mr Coyne has worked for the union for over 20 years. Before that he was a shop worker at Sainsbury's in his home town of West Bromwich.

He comes from a long line of so-called "moderate" Midlands trade union officials and Labour MPs. They were instrumental 30 or so years ago in organising the counter-offensive within the Labour Party against the hard left, which culminated in the purge of Militant.

So Mr Coyne's challenge inevitably invites comparisons - is history about to repeat itself?

Power games

Gerard Coyne himself disputes that "moderate" tag. He says there is much about Britain under the Conservatives that makes him very angry. But one of his principal campaign themes will be that Unite should concentrate less on playing party political power games at Westminster and more on the interests and welfare of its own members.

But he cannot escape the suggestion that however non-political he may try to make it appear, his campaign to depose one of Jeremy Corbyn's closest allies is, in itself, intensely political.

So what are the chances?

As the incumbent, Len McCluskey is the clear favourite. But he suffered a serious setback when the Unite Now faction within the union, which has always backed him in the past, decided not to give him their support this time round.

Gerard Coyne believes the higher the turnout, say above 20%, the better his chances of pulling off a shock victory.

Image copyright Ian Allinson Image caption Ian Allinson's bid for the General Secretary post at Unite is supported by the Socialist Workers Party

Nominations do not close until the end of next month and there is already a third candidate in the field. With the support of the Socialist Workers Party, Ian Allinson bills his campaign as "the grass roots socialist challenge".

But this does still have the look of a predominantly two-horse race.

Dividing line

One clear issue between Messrs McCluskey and Coyne is defined by Jeremy Corbyn's support for unilateral nuclear disarmament. It is another echo of those epic Labour battles back in the 1980s.

Unite represents many workers in defence and defence-related industries. So Mr Coyne's supporters accused Mr McCluskey of "a betrayal" when he appeared to support the idea of a new generation of submarines which would not be equipped with nuclear warheads.

"That's ridiculous. It's like making pencils without lead in them," Mr Coyne told me when he joined me in our BBC Birmingham studios earlier this week.

I'll be asking Jeremy Corbyn what he makes of that, and if he is feeling the pressure of Unite's leadership contest.

You will be able to see my interviews with both Mr Coyne and Mr Corbyn during this weekend's Sunday Politics, which is back in its usual 11:00 slot on BBC One in the Midlands this Sunday 15 January 2017.

You will have to wait just a little longer for the result of Unite's leadership election itself. That's not due until Friday 28 April.