Vital Statistics on American Politics
Contains over 225 tables and charts on American politics dating back to 1788, including data on presidential, gubernatorial, and congressional elections.
Vital Statistics on Congress
Useful data on our first branch of government – in the election and composition of its membership as well as its formal procedure, such as the use of the filibuster, informal norms, party structure and staff." From the Brookings Institution and American Enterprise Institute.
U.S. Census
Probably the world's most authoritative source for demographic information, including voting registration data.
These are tools for finding information from the U.S. Federal Government.
Federal Government Watchdog Agencies
Congressional Research Service, Congressional Budget Office, etc.
U.S. Congress -- Sites and Documents
Official publications, sites, and more.
U.S. Presidential Documents
Official series, online repositories, and official sites.
This is almost certainly the most comprehensive database of Congressional publications, some dating back as far as the late 1700's. Includes the complete texts of hearings and most other documents. An interesting sub-set of the database provides legislative histories. (Law Library subscription.)