Why Keir Starmer is the Labour leadership candidate to beat, writes Robert Peston

If you believe betting odds, Rebecca Long-Bailey is a shoo-in to be Labour leader, after the shadow chancellor John McDonnell on Sunday anointed her as the candidate most likely to build on the Corbyn/McDonnell project.

But punters may be getting ahead of themselves.

For one thing, I am told that a YouGov poll of Labour members carried out in the week of the general election delivered the same result as a YouGov poll in July - namely it showed that Keir Starmer is more popular among members than any other candidate.

Angela Rayner - Long-Bailey's flatmate - looks set to run for deputy leader rather than leader. Credit: PA

Back then, 68% of Labour members said Starmer would be a very or fairly good leader, and only 12% said he would be a poor leader.

I am told that last week's poll showed that Starmer's popularity among Labour members is still of a similar magnitude.

By contrast, just 34% of them in July said Long-Bailey would be a good leader, and 21% thought her poor.

So Starmer came top both gross and net, in each case by a wide margin.

Which is presumably why the Long-Bailey campaign is so fast out of the traps - and why her supporters are saying that the next leader must have three qualities that Starmer lacks, namely the leader must represent a northern seat (he's central London), mustn't be a passionate Remainer and must be a woman.

All of which is to say that - appearances to the contrary - Starmer is the candidate to beat. And after the contest starts in January, the first big hurdle for them all will be the endorsements of the big unions.

It is widely assumed that Len McCluskey, general secretary of the most powerful union, Unite, will throw his union's considerable weight behind Long-Bailey, even though McCluskey and McDonnell have not seen eye-to-eye on everything in recent months.

Long-Bailey is seen as the candidate most likely to build on the Corbyn/McDonnell project. Credit: PA

And there is talk that the GMB will back her too.

My understanding is that McCluskey will call a Unite executive meeting in mid to late January, when all the candidates will be invited to pitch.

However, the endorsement of any union is mainly useful in getting a candidate off the nominations list and into the final ballot of members. In the end, the choice of leader will be made by individuals, namely, members, affiliated supporters and registered supporters.

So given that ultimately the decision in March about who leads Labour will be taken mainly by members, it is striking that back in July, Angela Rayner - Long-Bailey's flatmate and pal, who looks set to run for deputy leader rather than leader - polled better than Long-Bailey, as did Emily Thornberry.

Of the likely candidates, only Jess Phillips polled worse than Long-Bailey - because Phillips had been a consistent critic of the members' then darling, Jeremy Corbyn.

Phillips knows she has a mountain to climb, as does the other likely candidate, Lisa Nandy, who wasn't seen as a credible successor to Corbyn till more recently.