@Real Music,
The Republican vote is skewed toward older white people, and older retired people have lots of time to vote. The Democratic vote has many more poor and working poor, and many of them are working more than one job-they have little time to get to the polling place. Many minorities grew up not being terribly involved in the political process. Plus Democratic voters also tend to be young, and they too haven't been involved in the voting process either, they need the excitement and attention of a Presidential race to get motivated.
The upside of all this is that once somebody has voted a few times, they tend to vote more often. If they vote in a Presidential election a few times, at some point they will say, "Heck, I think I'll go down for the midterms". I don't know how many more Presidential elections the Democrats will need to get those voters into midterm voters, but at some point it is going to break that way.