So what is driving Burnham's current magnetism? In short, he is seen as a winner and Sir Keir is seen as a loser.

And not just any winner – a winner against Reform UK, in a part of the country where Reform did very well in the English local elections last month.

Without question, Burnham didn't choose an easy seat to contest to return to Parliament. But it is also true to say that he is very popular in Greater Manchester and his popularity remains largely untested beyond.

"It feels like lots of Labour MPs are dashing for a train that's about to leave the station, but they haven't any idea where it's going," is how one figure put it to me, suggesting there is still plenty the party and the country doesn't yet know about Burnham's prospectus for government.

He set out some ideas, in broad terms, during the by-election campaign, but – perhaps understandably – wanted to focus then on the local concerns that might help ensure he won the seat.

In a social media post reflecting on the prime minister's decision to stand down, he added that "people want to see progress on economic growth, cost of living, public services, housing and opportunities for the next generation".

Few, of any political persuasion, would disagree with this. How he would deliver it will now face intense scrutiny.

It is the lack of detail and potentially very narrow window to fill in the blanks that are causing jitters for some in the party.

It is why some Labour MPs are talking up the idea of trying to persuade Darren Jones, the chief secretary to the prime minister, to stand in a contest against Burnham.

His friends suggest that, on balance, it is probably unlikely that he would, but he isn't ruling it out at least yet.

The next key moment in all this, then, will be when Burnham decides to set out his stall.

A man who was, until the early hours of Friday, a mayor hasn't had much reason to expound in public on foreign affairs, for example.

But he can be sure, if he becomes prime minister, to spend a huge amount of time considering it.

So how might he approach a relationship with US President Donald Trump, for instance?

Would he be willing to find the money to provide the military with the money they claim they need to keep us safe? And if he was, how is he going to pay for it?

A handful of questions among the dozens and dozens anyone might ask about a man who could be leading the country as soon as next month.