This scenario involves a leap of imagination.
There is no guarantee that Burnham will win the Makerfield by-election, which will be hotly contested by Reform UK. Burnham would only need to stand down as mayor if he won the by-election.
But it’s still a big gamble for Burnham as he can only mount a challenge for the Labour leadership if he’s an MP.
But if he manages to pull that off, he would be able to argue he has proof that he could revive Labour’s deflated standing in the opinion polls and beat Reform UK in a general election.
In contrast, Sir Keir has a pretty poor record against Reform UK, which pummelled Labour in some of its historical strongholds in last week’s elections in England, Scotland and Wales.
Back in Parliament, Burnham could mobilise the necessary support to trigger a leadership contest and take that electoral evidence into the campaign.
In a head-to-head with Sir Keir, who has just lost hundreds of council seats to Reform, Burnham would have the wind in his sails.
If such circumstances come to pass, they could set Burnham on a course to No 10.