The thing you need to know about Greater Manchester Mayor Andy Burnham is that: Keir Starmer felt so threatened by him that he blocked him from running in a February by-election.
It is no wonder why.
Mr Burnham consistently ranks as one of the most popular Labour politicians with the British public - and he is the favourite of a huge chunk of the parliamentary party too.
A 2021 YouGov poll found 69% of Labour members preferred him as leader over Mr Starmer.
And Mr Burnham has made his leadership ambitions pretty clear in recent years.
By contrast, Health Secretary Wes Streeting, who resigned yesterday but stopped short of formally launching a leadership challenge, was always seen as a mere complication.
Yes, Mr Streeting could probably garner the 81 signatures needed to trigger a contest.
But few thought he had enough support among Labour members to actually win such a contest outright.
No one thinks that about Mr Burnham.
So, if the Greater Manchester Mayor can make it back to parliament - via the by-election triggered by the resignation of Labour MP Josh Simons - he will pose a serious threat to the prime minister.
The fact that Mr Simons is a former ally of Mr Starmer only sharpens that threat.
Mr Burnham's allies are now framing the news as a piece of clever political theatre.
The seat he has chosen - Makerfield in Greater Manchester - voted 50% for Nigel Farage’s Reform UK party in last week’s local elections.
Labour got just 23%.
The supposed pitch, in other words, is that Mr Burnham is going on the offence against Reform - while Mr Starmer has only ever been accused of playing defence.
That argument would be more convincing if Mr Burnham hadn’t spent the past few weeks quietly searching for an MP to vacate a so-called Labour safe seat - one where he’d be all but guaranteed to win.
No one was willing to give him that.
But, be that as it may, Mr Burnham now has the opportunity to prove that he can take on Reform in their newest stronghold.
And Mr Farage, for his part, has wasted no time accepting the challenge.
"We look forward to the Makerfield by-election," he posted on X.
"Reform will throw absolutely everything at it," he added.
So it will not be easy.
The signs, however, suggest Mr Burnham will at least be allowed to try.
ITV News reported last night that Downing Street will not seek to block him from standing in Makerfield - a striking shift from February, when it did exactly that.
It is the clearest signal yet that Mr Starmer no longer feels he has the strength to fight Mr Burnham on every front.
Even if he does make it back to Westminster, Mr Burnham’s road to Downing Street is far from clear.
He has run for the Labour leadership twice before - in 2010 and 2015 - and lost both times.
His potential rivals would include Mr Streeting on Labour’s right and Energy Secretary Ed Miliband on the so-called soft left.
But he has one thing none of them do - distance from Mr Starmer’s unpopular government.
That makes him an appealing candidate to take the fight to Reform and Green Party.
That point is likely to be underlined by Labour’s deputy leader Lucy Powell, who is expected to use a speech today to publicly back Mr Burnham’s by-election bid and to say that he, Mr Streeting and former Labour deputy leader Angela Rayner should all be "key players" in the party’s future.
From the deputy leader, that is as close to writing Mr Starmer’s political obituary as you can get.
And so, for Mr Starmer, the writing seems to be on the wall.
Among Labour’s most senior figures, his departure now feels less a question of if, than of when - and how.
His only solace is timing.
A by-election in Makerfield would take, at minimum, six to eight weeks to hold.
And, in the meantime, no one is going to trigger a leadership contest.
The prime minister has, in other words, bought himself the rest of the summer.
The question now is whether he can do anything with it.
Few in Westminster believe he can.
