Yep, and Bush has already been out once for Kennedy. Don't think that the Dems won't be quick to point that out.
It may simply be better for Kennedy (and those candidates in more 'Rat-leaning states) to run their own races without bringing in the President (and in more GOP-leaning states, do the opposite). It's like the old LBJ tactic, he used to ask his Senators (in order for them to win reelection), "Do you need me to be for you or against you ?" Whichever position worked to get them elected was what would be used. You can technically run against the administration as a Republican without being a liberal, just trot out the old "I'll vote with 'em when they're right, and against 'em when they're wrong."