Non-voters are much more likely to be Democrats than Republicans.
This is largely due to demography: rich people vote much more than poor people, and old people more than young people. But even among people of the same age and income, those who lean Republican are more likely to vote. There are various theories as to why this is so. The one that resonates with me is that people lean Democratic because they want things to change in some dramatic way -- this would be especially true of those who pay little attention to politics -- but no matter who wins the election, things never change in the way that they want. So they say voting makes no difference.
It seems to me that if the Congress actually matched the ideological profile of the nation as a whole, as opposed to the voting part, the mandate would be there for a bunch of big changes: single payer health care, for example. But since poor people don't think their votes make any difference, they don't bother to vote, and the things that would help them don't happen.