Statistical Abstract of the United States
The Statistical Abstract offers statistical information (in PDF or Excel form) for a wide variety of topics, including elections.
Relevant tables for researching voter turnout (all in Section 7):
Table 416: Participation In Elections For President And U.S. Representatives: 1934 To 2012
Table 417: Resident Population Of Voting-Age, Total Votes Cast, And Percent Casting Votes--States: 2010 To 2012 [As of November]
Table 418: Voting-Age Population--Reported Registration And Voting By Selected Characteristics: 2000 To 2012 [By Age, Sex, Race, Region, Education, And Employment Status, Presidential And Congressional Election Years, As Of November]
Table 419: Persons Reported Registered And Voted By State: 2012 [As Of November]
Table 420: Reported Voting And Registration Among Native And Naturalized Citizens By Race And Hispanic Origin: 2012 [As Of November]