It is often said that Brexit cuts across the political parties.
That much is obvious - but at what point does it cut so deep that it severs party allegiances completely?
Splits have long been anticipated, but on the Labour side, we may not have long to wait.
I've spent the past few weeks speaking to those who are considering leaving the party, to those who are desperate for them to stay, and to those who'd happily see the back of them.
On the basis of those conversations, this is how I think a split could now play out. (Though I'm adding the disclaimer that this has all been anticipated before, only to prove an anticlimax.)
- The Crunch Moment
When is enough really enough for Labour MPs thinking of splitting away?
Well it now seems it could be the week beginning the 25th February.
That Wednesday, MPs will get another say on the PM's approach to Brexit, voting on a range of amendments.
And I understand half a dozen Labour MPs are planning to submit one calling for a People's Vote (which hasn't been sanctioned by the official campaign itself).
If - or more likely, when - Jeremy Corbyn refuses to back that amendment, the crunch moment could come.
- How many will go at first?
I have spoken to several MPs considering their future, and there is a core of half a dozen who are almost certain to leave.
Chuka Umunna's name is already out there, along with Angela Smith, Luciana Berger, Gavin Shuker and Chris Leslie.
But there are others too who could be persuaded.
Some of the breakaway MPs are already facing deselection by their local parties and in that sense have little to lose by jumping ship.
- But can they persuade others?
The bigger question is how many more MPs they can peel away by tapping in to three main sources of disillusionment in Labour:
1. Jeremy Corbyn's leadership and political beliefs 2. The Labour Party's troubles with antisemitism 3. Brexit, as discussed above
Each factor plays differently for each individual MP and it is wrong to give one reason particular weight over another.
But dozens of them are certainly frustrated about Jeremy Corbyn's reluctance to back a People's Vote, which could form the most obvious basis for a block breakaway at this stage.
However, as one Labour MP closely involved in the People's Vote campaign insisted to me, "most of us refuse to give in."
For many, abandoning Labour - for whatever reason - would be admitting defeat and surrendering their party to other forces.
Many MPs have a deep personal connection to Labour, having spent a lifetime attending the local Labour club and knocking on doors.
The Labour brand is arguably stronger than any other in British politics and leaving would feel like a huge upheaval.
And there are other complicating factors too.
One Welsh MP told me it was unlikely that any from Wales would breakaway, because Labour is in government there.
How would their ministerial colleagues in Cardiff Bay feel if MPs abandoned the party and humiliated them?
- What would they believe in?
The issue isn't just what MPs would be running from, but what they'd be running to.
What would these breakaway MPs do with their new-found freedom?
From my conversations, it's clear that those at the core of any breakaway do have a rough plan.
There has been discussion of some form of manifesto, or at least some key principles.
Generally speaking, they would stand on a centre-left ticket, with social democratic policies.
That could appeal to the likes of the Lib Dems and some Tory MPs, with talk of trying to peel them away too.
But I understand the big sticking point is the party colour.
Red? No. Blue? Taken. Yellow? Also taken. Purple? Was taken, now more available, but not a great idea. Brown? Hmm. It's genuinely tricky.
- What would the damage be?
One shadow cabinet source loyal to the Labour leader ticked me off this week for describing all this as a 'split'.
"No Paul, a split sounds like it's down the middle, this is just a handful of MPs," he told me.
He was relaxed about a breakaway, believing departing MPs would struggle to establish a new party that would pose any real threat.
Indeed, some have even suggested to me that it would be 'good riddance' - with the rebels gone, it'd be a chance to unify Labour at last.
Of course, the historical precedent everyone refers to is the SDP in the 1980s, which broke away from Labour in not dissimilar circumstances.
After initial electoral success, it ultimately failed to gain a foothold as a serious party, later merging with the Liberals to form the Liberal Democrats.
But this is to ignore the fact that the SDP contributed to Labour suffering 18 years in opposition, with the left divided and the Conservatives given a free run.
It is not an exaggeration to say that some MPs - only a few - are so horrified by the state of their party that they are partly motivated by a desire to keep Jeremy Corbyn out of Downing Street.
And by splitting the left-wing vote - in an era when elections are tight and parliaments are often hung - they could deny him the keys.
- So how likely is it?
A breakaway now seems inevitable.
The timing is still uncertain.
Perhaps it'll be the 27th, perhaps they'll wait a little longer.
But something huge would have to change to persuade those half a dozen MPs that they have a future in Labour.
As for their future outside of it? They're willing to take a gamble.