for the total US, or to select data by State, County, and City:
http://quickfacts.census.gov/qfd/states/00000.html
for a Congressional District, use:
http://fastfacts.census.gov/servlet/CWSFacts?_event=Search&geo_id=50000US2502&_geoContext=01000US%7C04000US25%7C50000US2502&_street=10+Main+Street&_county=&_cd=&_cityTown=&_state=04000US25&_zip=01757&_lang=en&_sse=on&ActiveGeoDiv=search_by_address&_useEV=&pctxt=fph&pgsl=500&_content=&_keyword=&_industry=
