Public Mapping Project: Read the discussion about how to reconcile Census and Election data, which can be a massive undertaking.
The Harvard Election Data Archive contains information on election results, voting behavior, and electoral politics. The core data comes from state, county, and district level election returns for all recent state and federal elections in the United States.
The Record of American Democracy (also from Harvard) provides precinct election data for the US from 1984-2001 and merged election data with Census data at the Minor Civil Division geographic level. Illinois data for years 1987-1997 only.
The Stanford Election Atlas has a visual map showing who won each precinct in the 2008 presidential election.
Find Voter Turn Out rates from the United States Election Project at George Mason University.
See this excellent presidential election results map that shows red states and blue states re-sized according to their populations and electoral votes.