Washington and Lee University
These first two sources are very similar in structure and purpose -- they provide authoritative, non-partisan descriptions of political and socioeconomic conditions in each U.S. state and each Congressional district, as well as information on each member of Congress.
Available online. National Journal.
Look for the "Browse the Almanac -- States and Districts" option for date and an excellent narrative on each state's political history and analysis. You can see similar analysis of each Congressional district by clicking on the name of each member of Congress.
Available online. Congressional Quarterly.
This resource does not have the state-wide narrative that the Almanac does, but the entries for each senator and congressman include political analysis. Each governor entry includes some state-wide data. CQ also produces:
REF JK 1967.A8 v.26
Comprehensive data collection; unfortunately, not available online. (earlier editions available)
The FEC is an independent agency which is primarily responsible for regulating campaign finances, but it also disseminates some electoral data for the 2004 and 2006 national elections.
REF JK 1341.C65 2003
Latest edition of this Congressional Quarterly compilation provides a description of the political, economic, and demographic characteristics of each Congressional district in the U.S.
VCS is a private firm which builds and markets files of voter information for political campaigns and other users, although it is a little unclear how current the data is and why some jurisdictions are not included.
Still, the free portions of their database are useful for data on voting in states, congressional districts, and counties. (Start from the map and then look for the "Statewide Report" option.)
Available online.
Data on presidential, congressional, and gubernatorial elections.
Available online.
Congressional Quarterly publishes this collection of data, which includes a section on Elections and Political Parties.
The Census Bureau is the premier data-gathering organization in the U.S. and it actually is the source of a lot of voting and registration data that shows up in other compilations.
The Voting and Registration section of the Census Bureau site includes several noteworthy collections, most notably the biennially-published Voting and Registration in the Election of November...which includes both a narrative report and detailed tables. Most recent is 2006, with additional studies further down the page.
The Census Bureau also publishes the annual Statistical Abstract of the United States, the most recent of which (2008) includes this compilation of Elections data.
Available online
The Elections and Politics section includes data on apportionment, turnout, voting, and more, including counts dating back to the late 18th century. Do not overlook the excellent overview essay.
The Electoral College, under the oversight of the National Archives, primarily is a repository of historical electoral data, although they also have valuable information on the 2008 election, including an Electoral College calculator.
"Since 1920, the Clerk of the House has collected and published the official vote counts for federal elections from the official sources among the various states and territories."
University of Richmond site provides "cinematic & interactive maps, and analysis, of the Presidential elections in the US from 1840-2004. This unique resource focuses on election data to the county level (rather than state)..."
REF JK 1967.R87 2001