Depending on how negative his relations are with Clinton by the end of the primaries, I honestly believe that he could pull a Eugene McCarthy and run as an independent in the general election.
McCarthy didn't run as an independent in '68, he ran in '76. despite hating Humphrey more than he hated Carter (I'd suspect).
I doubt McCarthy left Chicago in '68 physically capable of running a campaign any further. something had died there, and even Gene McCarthy, hardly a radical of any sort, could sense it.