Awhile back Chait argued that they are. Brendan Nyhan looks at the research:
Democrats have been less enthusiastic relative to the other party in the first midterm under both Clinton and Obama than Republicans were under Bush, but it's important to keep in mind that the 2002 election is an outlier due to 9/11.
By comparison, 1994 and 2010 were extremely unfavorable electoral environments. In more favorable conditions (principally, a booming economy), we see that Democrats were relatively more enthusiastic for Clinton in the 1996-2000 elections than Republicans were for Bush in 2004-2008. It's unlikely that Democrats will close the enthusiasm gap with Republicans in this election -- the conditions are just too unfavorable -- but the historical record doesn't indicate that they are incapable of enthusiastically supporting a Democratic president.