Immigrating to the United States in 1886, Hillquit joined the Socialist Labor Party in New York and became active as a union organizer; in 1888 he helped found the United Hebrew Trades. He earned a law degree from the University of the City of New York (later renamed New York University) in 1893. As a consistent advocate of Socialist reform within the constitutional framework, Hillquit came into conflict with more radical elements in the party led by Daniel De Leon. When the party later split, Hillquit's more moderate faction joined a coalition of Socialist groups under the name Social Democratic Party, which in 1901 became the Socialist Party. Hillquit emerged the party's chief theoretician and tactician. As such, he was largely responsible for the party's steadfast policy of pacifism during World War I, a position that drew widespread criticism after the U.S. entry into the war. With passage of the Espionage Act of 1917, Hillquit devoted much of his energies to defending Socialists in court. Hillquit ran unsuccessfully for Congress five times and for mayor of New York City twice. In 1924 he led his party in its support of Robert M. La Follette, presidential nominee on the Progressive ticket. Among his books are History of Socialism in the United States (1903), Socialism in Theory and Practice (1909), and the autobiographical Loose Leaves from a Busy Life (1934).