I think most can agree that in Washington, time served generally equals
power, status, and a certain amount of gravitas (as defined in DC), be
it actually earned or not. That said, I've heard that one of the
reasons Obama is so keen to carry on with the debates is that he has
little or none of the above, even in his own party. Sure, he may be the
nominee and he may be the next POTUS, but these folks hope to be
"working" in DC (and "working" with each other) regardless of November.
Because of that, the seniors aren't interested in including any junior
****-ant and that's been made clear to him, so he doesn't want to be in
DC and be seen to be marginalized, whereas McCain (and Biden) are among
the senior folks and will be a part of things. I have nothing that I
can post to prove it, but it is an interesting perspective...

R