Pat McCarran was the chairman of the Senate Internal Security Subcommittee that investigated the administrations headed by Franklin D. Roosevelt and Harry S. Truman. In September 1950 he was the chief sponsor of the Internal Security Act. This legislation required registration with the Attorney General of the American Communist Party and affiliated organizations.
In June, 1952, Pat McCarran and Francis Walter instigated the passing of the McCarran-Walter Act that imposed more rigid restrictions on entry quotas to the United States. It also stiffened the existing law relating to the admission, exclusion and deportation of dangerous aliens as defined in the Internal Security Act.