Paul Krugman sees a glimmer of hope for the 2010 midterm elections. There are two reasons I don't share his optimism. First, one cannot be very optimistic in the first place about the possibility of snatching victory from the jaws of defeat: by its very nature, such an outcome is the essence of unlikely.
More importantly, in 1948 (to which Krugman compares the 2010 elections) the Democratic party was sincerely and publicly committed to the New Deal. Today's Democratic party does not, however, have a sincere and public commitment to anything but the most expedient "centrism". A narrow Democratic win in 2010 would definitely be embarrassing to the Republicans and might ease the worst of their batshit craziness, which is not the worst possible outcome.
Strongly exceeding expectations (a narrow victory where a severe loss was expected) can itself establish a mandate, but the Democratic party doesn't have anything to establish a mandate for.