The Transport Workers Union of America came into existence in 1934 as a result of organizing efforts by a core group of workers on New York's Interborough Rapid Transit system, with assistance from Communist Party activists. After slow gains among NYC subway and bus workers and a brief period of affiliation with the International Association of Machinists, the union affiliated with the CIO in 1937 and was granted a broad jurisdiction over all categories of transport workers. Under the dynamic leadership of President Michael J. Quill and a group of organizers who shared his Irish nationalist background and radical outlook, the union soon grew to more than 40,000 members and began organizing nationwide. The largest and most powerful locals of the TWU have been Local 100 (NYC) and Local 234 (Philadelphia), but by the post-World War II period the TWU also had a substantial presence in Chicago, Detroit, San Francisco, Miami, New Orleans, Omaha, and other cities and towns across the nation.