iPOLL is the largest source for opinion polling data. Data covers 1935 to the present on a broad range of topics. Questions can be searched by keyword, topic, date, or source.
The database includes polls from many sources: major news organizations (CBS, ABC, Associated Press, etc.), public opinion research centers (Gallup, Harris, Pew, etc.), and major academic and commercial pollsters. Only polls of the United States population at the national level are included, but international datasets are available for immediate download.