The government produces a lot of valuable information for researchers in United States History in the mid-Twentieth century. There are laws, court cases, reports from different government agencies, congressional or legislative hearings and reports that can be extremely valuable to historical research.
American Memory
American Memory is a series of primary source documents at the Library of Congress and other Libraries about different aspects of American history, put together into different online collections
The U.S. Census Bureau counts each resident of the country, where they live on April 1, every ten years ending in zero. It includes populations of all states in the country, plus populations of counties and cities. It also includes agricultural data, industrial/manufacturing data, and other information, such as number of schools by state. The link below takes you to a page where you can gain access to any census from 1790 to 2010.
Law