Democrats have convictions. They know what to do with power when they get it and how to isolate, even punish, any member of their party who dares to take a different position on an issue.
Actually, Republicans do the same thing (Ask Ted Cruz, Rand Paul, any Tea Party candidate, anybody who runs against John Boehner or Mitch McConnell, etc.), Republicans aren’t losing for lack of conviction. They’re losing for lack of any motivation other than hanging on to power as hard as they can.