Some conservative media wit ought to stick Kerry with the title "Senator 13%". I think it might catch on.
Also, if he refuses to resign his Senate seat, conservatives can point out that Kerry probably doesn't really expect to be elected President. Therefore he holds on, Lieberman style, to his safe Senate seat.
Another appellation that fits would be "Senator from Massachusetts for Life".
How 'bout the putative nominee is also a putative Senator.
And some reporter should ask him if he plans to be absent from work as much if he becomes President.