Gordon Brown is not going to resign; no matter how low they go in the polls, or how much local or by-elections they lose or how much he destroys the country. He slogged through the Blair years, yearning for this job and he will not go down in History as one of the shortest serving and most useful Prime Minister's ever.
However ... if he were to fall under a (political) bus, what is clear is that it's Jon Cruddas who will carry to flag of real Labour forward. His excellent showing in the Deputy Leadership contest - coming from nowhere to winning on first preferences - plus the dignified way he has handled himself since has put him in poll position. With all of the other Deputy Leadership candidate pretyt much humiliating themselves (including the eventual winner), he still holds a place in the heart of the Labour Party membership and the Trade Union movement if not the PLP.
So, if its Cruddas then who will he be against? I believe that the deal will be done to ensure only one candidate emerges from the Blairite right of the party; they won't want to split the vote and will want to be seen to be united. They may find themselves against a self-styled "unity" candidate like Jack Straw, but the Milliband-Balls-Burnham-Purnell alliance will be at work.
If I were in that alliance, I'd be working out who is going to be doing the challenge in the next 2 years - because they'll need all the traction they can get to beat Straw out of the traps and beat Cruddas to the votes.