Since the mid-19th-century emergence of the current two-party competition, no party holding the presidency has ever won control of the House in a midterm election.
Since the Civil War, the average turnout in presidential elections has been 63 percent and in midterms 48 percent.
The decline comes mostly from the party holding the presidency.
Three crucial components of Obama’s coalition — unmarried women, minorities (more than 40 percent of Obama’s 2012 vote) and young people — are especially prone to skipping midterms.
In the seven midterms since 1984, voters younger than 30 averaged 13 percent of the midterm vote, down from 19 percent during presidential years.
http://www.washingtonpost.com/opinions/george-will-politics-by-the-numbers/2014/01/03/bbab651a-74a6-11e3-8b3f-b1666705ca3b_story.html