I know Dean isn't making a stand on principle, so it must be simple cowardice.
Seems to me Dean's decision to pass on appearing with Mehlman is a direct contradiction to all the Major News Organization's propaganda that "Bush is weak"; "Majority are against the war in Iraq now"; etc. If all this pap were true it would be an ideal time for Dean to go Mano-y-Mano with Mehlman publicly.
But the again, as has been discovered in the last few days, there have been numerous requests for the President to appear at fund raisers with candidates. Now why would they ask for him if he was so weak, and despised? Maybe the news being reported is, well, untrustworthy?