i think Kerry is hedging his bets... everybody knows Edwards is not going anywhere.. South Carolina coming up ought to make that perfectly clear. Edwards didnt win his own state before, he likely wont this time.
So Kerry is hedging that the US doesnt want another Clinton in the White House.. and the GOP doesnt have an obvious front runner yet.
He's hedging that Obama could walk away with it .. and he might finally get into somebody's cabinet for once in his life.
Its a long shot, but I would imagine that's his thinking..