Chris Mason: Another crunch moment for Starmer as he pleads with Labour MPs not to topple him

It feels like the prime minister has to give the speech of his life today.

Those within the Labour Party who want to see him succeed acknowledge that you can't change everything in one speech.

But it is clearly imperative for Sir Keir Starmer to try to calm down a party that is hurting and anxious.

Many Labour MPs have spent the weekend observing the politically scorched earth around them locally – their friends and colleagues in local and devolved government wiped out. There are fraught emotions and there is anger.

And for the last few days now there has been the drip, drip of revolt, with Labour MP after Labour MP coming out publicly to say Starmer has to go.

With every one, a little more of the prime minister's authority drains away.

Incidentally, don't underestimate what a big deal it is for any individual MP to go over the top and say their boss should go – not least because, for now at least, those that have done so are a tiny fraction of the total number of Labour MPs.

And it was his name up in lights as their leader when many of them won their seats for the first time, and often in parts of the country where Labour rarely if ever win. So to say now, out loud, that you think he is a dud is a big deal.

Wherever you look in the Labour Party right now there are knots of anxiety.

Firstly, there is anxiety in Downing Street, of course. They are acutely aware of what is at stake.

Secondly, there is anxiety among the potential challengers, weighing up if, when or whether to go for it. Timing can be everything: get it right, and the premiership can be yours. Get it wrong, and what might be your only chance to be prime minister is gone.

Thirdly, there is anxiety among the many, many Labour MPs keeping their heads down and who really don't want the prime minister to leave right now, nor for there to be a leadership contest.

Then there are those who would like Mayor of Greater Manchester Andy Burnham to be Labour's next leader and so don't want a contest right now – because he needs time to firstly find and then win a Westminster seat, having been blocked from standing in one just a few months ago.

So what happens after the speech tomorrow? How do Labour MPs react? Does Catherine West, the former minister who has said she is willing to challenge the prime minister to try to force a contest, decide to back down, or press ahead?

Does the prime minister manage to put people off challenging him, at least for now?

Or is there a flood of anguish that leaves his position untenable and tempts one of the challengers to go for it?

Health Secretary Wes Streeting, in particular, faces a massive call in the next couple of days. He has said he won't challenge Sir Keir, but is prepared to make his case if it becomes clear the prime minister is a goner.

So does he go for it, or not? Some who would like to see him replace Sir Keir think this might be his very best chance, before Burnham can get back to Westminster.

It is worth emphasising that it is not easy to dislodge a sitting prime minister who doesn't want to budge and, up until now at least, Sir Keir has given every indication he wants to stick around.

But what a moment he confronts and his party confronts.

The Labour Party is in a glum swirl right now, where no one can be certain what will happen next.

Whatever does – or doesn't – happen will have consequences for us all.