He's expendable. If he wrests the nomination away from Howie, then he helps the Clintons keep control of the Rat party. If he loses, his delegates still can keep their leadership (McAwful) from total meltdown. Either way, they can blame him for any losses.
And, if he should win, by some wild stretch of the imagination, or the untimely demise of President Bush, well, there's still the files on him to keep him in line. If he ever got close to the nomination, the Clintons would make damn good and sure that somebody they've got the goods on gets the Veep nod, then that person gets moved out after the 2006 NY Senate election, and Hillary becomes VP. Clark either steps down in 2008 for health reasons (dead being one possible reason), or his file gets made public, and he drops his run for re-election.
For the Clintons, its win-win-win, no matter how you look at it.