... if he were to mention - now - that - in the event of his not having a good election (he can choose his formula - not getting the highest share of the vote / largest number of seats) that he would regard it as a signal that he can't carry on as PM and that he would step down as Labour's leader.
It would be a bombshell at this stage of the election, and it would very probably increase Labour's vote and give the Lib-Dems a stronger case for hooking up with Labour rather than the Tories in the event of a hung parliament.
I can see no arguments against this and I'm surprised that it's not an option that is being discussed more widely. It's not an endorsement of tactical voting. It shouldn't carry any risks to Labour's current campaign and it isn't a hostage to fortune of any kind.
(I know I've said this before but it needs repeating).