Highly respected collection of reliable data on elections and voting, as well as fund-raising, demographics, etc.
The Census Bureau regularly conducts surveys on "voting and registration by various demographic and socioeconomic characteristics" and provides both recent and time-series data.
High quality data from surveys on voting, public opinion, and political participation.
Provides informed analysis of election polling as well as a model that aggregates a multitude of polls to forecast the election
Gallup.com provides articles that summarize the results of recent, selected polling data. Includes some international polling.