If Democrats keep the House of Representatives and maybe even the Senate it will be because of a fear of the Republican Party. There are stories about planned investigations of the Obama White House, and also a general sense that the extreme right wing would take over.
I discount the reality of these. In fact, we need more conservative leadership, but I doubt the GOP would really bring that.
The problem for Democrats with this motivator is that it won't work until it is clear Republicans will do tremendous harm. First, the public has to be convinced Republicans are going to take over, and that this potential is bad.
This can only really occur in the final 10 days of the campaign.
The momentum at that point could be too much to overcome.
The 2000 election showed what can happen after a long campaign in just 1 weekend with Al Gore closing the gap on Bush in the polls, and eventually winning more votes.
But that is a longshot.
I expect "unlikely voters" to rule the day. The key is whether on election day and the preceding week, Democrats can get them to go to the polls. These are people who will only care long enough to pull a lever. And you had better hope the line isn't too long.