The TNR Gambit
thinks that the TNR endorsement of Lieberman cements their irrelevance. Clearly, a Dean endorsement is out of the question, but Clark seems like the obvious choice to me.
My feeling is that they want to pick someone who will definitely lose the primary and then hold out for a general election loss by the Democrats. It only works as an "I told you so" moment if they constantly warn that any other choice than theirs has no chance.
This will further two goals of TNR: (1) sabotaging the Dem chances in the general (they are, after all, Republicans) (2) gaining more influence in the Democratic party. The problem with endorsing Clark is that he might win the primary, and then they'd be stuck having to help him, and their fortunes would be tied to his -- if he lost, they'd have to explain why Dean would not have done better.
Now that TNR has endorsed Lieberman, the worst thing that could happen to them is that he actually wins.