The Nixon-Mundt Bill, HR#5852, was passed by the House on May 21, 1948 by 319 to 58. It required that members of the Communist Party of the United States register with the Attorney General. It also said that federal employees could not participate in the Communist Party and could not "knowingly hire" any Communist Party members. Finally, it denied passports to Party members to restrict their travels. As there was absolutely no benefit to registering with the government, many communists remained "underground" and never registered.

Also called the "Subversive Activities Control Bill", it was engineered by Representative Karl Mundt (SD) and then-Representative Richard M. Nixon (CA), who was even called "one of the greatest patriots in all American history" by Representative Ben F. Jensen (IA). Arguing for support for the bill, Mundt is quoted as saying, "communists in this country are guilty of sabotage, propaganda against the interests of the United States in time of war, physical abuse during elections (and murder) plus hundreds of crimes such as draft dodging, passport faking, perjury and lesser crimes."

The bill was fought by the National Committee to Defeat the Mundt Bill. However, many of their arguments against the bill were just as extreme as those arguing for the bill: the Committee said that, if put into effect, the bill would create legal fascism and pave the way for the restriction of civil liberties of any other group the government deemed a threat. The Committee was not the only group against the bill; Stephen I. Zetterberg, Nixon's congressional election opponent, voiced his opposition to the bill, and even anti-communist public figures, such as William O'Dwyer (Mayor of New York), did as well.

The bill died because the Senate never took action, but it was expanded and reintroduced two years later as a response to President Truman's call for new legislation dealing with communists. Added was a provision making publications or broadcasts produced by an organization labeled as communist specify that they were sponsored by a communist organization. The second time around, it passed in the House 354 to 20, and garnered much attention in the Senate. Communism was seen by many of the bill's supporters to be an international conspiracy to overthrow the Constitution and democracy as we know it, so the bill was not deemed unconstitutional because it was "protecting" the Constitution. Those against the bill considered it thought control or "legislat[ion] in a spirit of hysteria."

After the bill passed through the House the second time, the Los Angeles County Board of Supervisors passed an emergency ordinance "requiring all Communists or Communist sympathizers...to register at the sheriff's office after September 1st or face a $500 fine and six months in jail for each day's failure to register."

The bill itself did not pass the Senate, but after weeks of debating, Senator Pat McCarran (NV) drafted another one borrowing many of the provisions of the Nixon-Mundt Bill that, with the addition of the Emergency Detention Act of 1950 (called the "concentration camp clause"), passed by a large margin. The McCarran Act reads: "The detention of persons who there is reasonable ground to believe probably will commit or conspire with others to commit espionage or sabotage is, in a time of internal security emergency, essential to the common defense and to the safety and security of the territory, people, and the Constitution of the United States of America." Punishment was a fine not more than $10,000 or ten years in jail, or both.

Upon the bill's passing, communists vowed never to register. With Nixon's help, the McCarran Act became known as the Internal Security Act of 1950. This act was so harsh that even President Truman questioned its constitutionality, saying, "We must be eternally vigilant against those who would undermine freedom in the name of security," reflecting the 200-year-old words of Ben Franklin. Truman went on to veto the Internal Security Act, but his veto was overridden with an 89% majority vote. The battle over this bill strongly reflects the dispute over the USA PATRIOT Act: just replace "communist" with "terrorist" and "Internal Security Act" with "USA PATRIOT Act."