The same problem they've been having for awhile:
They suck.
In all seriousness: the refusal of many Democrats in vulnerable seats to stand by and defend Obamacare. Now yeah you may hear BUT THE POLLS! but Obamacare was really a case of the narrative being set by the right that IT'S SOCIALISM MANNNNN and FREE ABORTIONS BABYKILLERS that was repeated constantly without a strong enough pushback from Democratic congressmen and officials that was why it was unpopular back then. If Blue Dogs and what not had grown a spine and defended Obamacare, screw the polls, they might have lost their elections anyway but there at least would be the narrative that Democrats are for stronger government, not that they are somewhat receptive to the idea of stronger government until they hear someone say that their ideas are bad.
Of course, voting against a bill with that much push and support from both activists and the party leadership, going on the campaign trail to brag about how much you opposed it, and then not really going into details about any alternative plans you have that would look much better against your Republican opponent, certainly does not seem like the sort of campaign you would run to win an election.
Just saying.
That, and the Democrats were bound to lose seats. They had probably the strongest congress since the 1970s (60 Senate Seats, 256 seats) and the honeymoon period for Obama ended after about 5-6 months into office. Once that period was over his poll numbers gradually went downhill. With a 60% Senate/57%ish House Democrats were probably going to lose at least a dozen house seats in Blue Dog areas and several Senate seats in red states.
All things considered the Senate results weren't horrible. The Democrats still had a healthy enough margin in the Senate to keep the GOP at bay. The House was where the real massacre happened. It wasn't until October of 2010 that people began to realize just how big of a wave might be in store for the Democrats. Like, I think most people knew the Democrats might lose the House due to how many conservative and moderate members were in the caucus. But getting below 200? That would have to take effort.
