i'm not sure that's fair comment; aside from boosting his name rec and creating a public narrative for himself, there was almost no benefit for McCain.

it would have been a matter of grooming some other Dem presumptive(s) in the public eye and making sure McCain remained in the shadows. and, as someone else mentioned above, he'd end up burning as many bridges as he builds.

he'd probably have to actually become a Dem to make a hypothetical shot at the Office work.