This chart from Alan I. Abramowitz, The Disappearing Center (Yale, 2010) deserves attention:

Turnout has risen for people who strongly identify with parties and has fallen for those who do not. Or … people who vote have become more attached to parties, while people who don’t vote have moved into the Independent column. Or some of both. As a result, an Independent is likely to be a non-voter and a voter is likely to be a partisan. This was far from true in the 1950s. I am not sure whether the trend is good or bad, but it is an important explanation of politics today.

(All the turnout rates shown above are exaggerated because of social desirability bias, but the relationships among partisanship, year, and likelihood of voting should be valid).