Here's a fact you can research on your own.
VAP is Voting Age Population. It represents 'National Adults'.
In 2000 and 2004, 55% of the VAP voted.
In 1998 and 2002, 36% of the VAP voted.
From what you are showing me, there is a new trend. Low voter turnout, back in the 80s, used to benefit conservatives. But now, it appears that low voter turnout benefits liberals. Is that about the gist of it?