The main reason the Dems got crushed was that the voters they rely on are not reliable.
We have the young and minorities who only seem to vote in great numbers during Presidential elections.  The reliable voters are labor who have been successfully demonized by the right with the help of the media.  Newspapers run articles like this one awhile back on Labor Day.  “Are unions evil”  Unions are blamed for the downfall of cities and the debt that states have.  The Democratic party doesn't fight back on these issues.  In Wisconsin where the Governor stripped public unions of most of their rights, very few national Democrats spoke out or came to Wisconsin to stand with the workers.
President Obama should have shown up in Wisconsin, but of course he didn't want to take sides.
The media which has been painted by the right as a liberal bastion is a fantasy that people have bought into.  The networks, and newspapers are owned by corporations who are dictating editorial policy. All you have to do is watch any so-called news program on any network especially on the weekend and who do you see?  John McCain, or Lindsey Graham, Rand Paul, or Ted Cruz  Another example of this is the paper in Duluth Mn.  Duluth is a relatively liberal area.  The paper endorsed almost all Republicans during the recent election.  Of course if you look at the actual endorsements none of them came from the local editorial staff.  They all were from the corporate owners from out of the state. The paper is doing a disservice to the community. One of the states television channels wouldn't run some ads by the Democrat in a congressional district claiming there were  false claims made.  The thing is the owner of that television station was a supporter of the Dems opponent, and a dyed in the wool conservative who only believes in fairness if it benefits him.  Basically he was picking and choosing which ads to air  based on his political bent.
Finally the democrats have some really bad candidates.  We had Grimes in Kentucky who refused to say who she voted for in the previous Presidential election. We had a candidate for Governor in Massachusetts who was an embarrassment and this was not the first time she screwed up an election. Who are the people nominating this person.  The list goes on and on.  The Democrats let the Republicans define them. They use gun-rights or abortion as a gun to their head.  They can't seem to refute that background checks are not gun control.  They never mention the fact that abortion will not go away even if it is repealed.  Back alley abortions will kill more women like it used to. They never push the Republicans on their warped reading of the Bible.
Democrats have to get tougher and quit trying to placate the Republicans