Partisan site, but a highly respected collection of reliable data on elections and voting, as well as fund-raising, demographics, etc.

The National Association of Secretaries of States provides links to each U.S. state's agency regulating voting and elections, usually with access to historical voting results and registration data. (You also can scan down the NASS homepage to "Find Your Secretary of State.")
A similar list is available from the U.S. Federal Election Commission.

Over 30 tables of data on elections and voting behavior.

The Census Bureau regularly conducts surveys on "voting and registration by various demographic and socioeconomic characteristics" and provides both recent and time-series data.

Data on elections and voting, campaign finance, public opinion, etc.

Detailed examination of the history of U.S. elections and the electorate through the 20th century.