Theresa May and Jeremy Corbyn are both on that Italian-Job bus dangling over the cliff, with gold bars at one end and survival at the other.
May wants to pursue her Chequers Brexit plan, even though doing so is alienating up to half her own MPs, True Brexiters and some erstwhile Remainers like Nick Boles (though in truth he has always been more Govean - or the agriculture secretary’s representative on earth - than europhile).
According to her senior colleagues, May will not turn back - even though continuing to negotiate with the EU on her Brexit scheme would deliver a deal even less palatable to the Davises and Johnsons than the White Paper policies she wrote and they used as reason to quit her cabinet (if there is a deal at all; the damning overnight verdict of the EU’s chief negotiator Barnier augurs ill for May’s plan).
Nor is it only her position as PM that could tumble over the precipice, if her Brexity critics make good on their threat to launch a coup.
Although all chatter is about fission of the Labour Party, the Tories are an equally fragile coalition of left and right, Brexiters and Remainers - and neither she nor any successor will be able to satisfy all her MPs.
So predicting whether the next MP to resign party membership will be Labour or Tory is a mug’s bet.
What is clear is that there are tens of Labour MPs whose shaky loyalty to Corbyn and their party is almost wholly contingent on a single decision being taken at Labour’s conference - which is whether all Labour MPs should automatically face a reselection decision before the next election.
With support for this proposition from constituency parties, Unite and the Corbyn-loving Momentum movement - whose people will dominate the floor of conference - there looks set to be a rule change that would put in jeopardy the parliamentary livelihoods of all but the most devoted adherents to the Corbyn cause.
The minority of so-called centrists still on Labour’s NEC tell me only Corbyn can intervene to block or postpone this rule change. But his closest colleagues tell me he cannot and will not do this.
If Corbyn plays the innocent and powerless bystander, he would end up with near absolute control of every tier of his party, from grassroots membership, to local councils to the House of Commons.
But at a price.
In respect of the Commons he would be the dearly beloved leader of a shrunken party: Labour MPs, many despairing at Corbyn’s alienation of the mainstream Jewish community and knowing they were about to be defenestrated by their Corbynista constituency members, would follow Frank Field and refuse to follow the Labour whip.
Or to put it another way, Corbyn’s desire to democratise Labour and put MPs in permanent thrall to members would accelerate the self-imposed exile of Labour’s right.
Which in turn would neuter Corbyn’s ability to hold the government to account.
With Corbyn and May both teetering on the brink of their own special brands of calamity, which of them will be first to utter the Italian Job’s final and resonant line of utter desperation - “hang on a minute lads, I’ve got a great idea”?