Some Democrats suggest that there is little substantive difference between Senators Obama and Clinton on the issues, so that we must be happy with either of them.   This view is seriously mistaken.   Others, concerned with disharmony in the party, argue similarly that strong appeals for either candidate, or against either, are overwrought and should be squelched.  This too is badly mistaken.   There are important reasons from a Democratic perspective not to put Hillary Clinton on the Democratic ticket this year, and they are worth fighting about passionately, though not hatefully.  Here are several of them: