The Democrats returned to power in the state in 1912, and subsequently New York seesawed from one party to the other. The reform programs continued to gain ground, however, and Democratic state administrations between World War I and II—those of Alfred E. Smith (1918–20, 1922–28), Franklin D. Roosevelt (1928–32), and Herbert H. Lehman (1932–42)—presided over a wide variety of reform measures. The reform programs emphasized public works, conservation, reorganization of state finances, social welfare, and extensive labor laws. Four years after Smith’s defeat in the 1928 presidential election, Roosevelt went to the White House. Lehman followed Roosevelt’s national New Deal program by instituting the Little New Deal in New York state. At the same time Fiorello LaGuardia, Republican mayor of New York City (1934–45), enthusiastically supported Roosevelt’s social and economic reforms.

The Republican party returned to power in the state in 1942 with the election of Thomas E. Dewey as governor (reelected 1946, 1950). Dewey had the immense task of coordinating state activities with national efforts in World War II, straining New York’s resources to the utmost. He also built upon the reforms of his predecessors, extending social and antidiscrimination legislation, and won a reputation for effectiveness that made him twice (1944 and 1948) the Republican presidential nominee.

During the governorship (1959–73) of Nelson Rockefeller, a Republican, state social-welfare programs and the State Univ. of New York were expanded, and a large state office and cultural complex was built in Albany. New York’s growth slowed from the 1970s, though, as the state lost its dominant position in U.S. manufacturing, and the older cities lost businesses and residents to suburbs or to other states.

As economic growth accelerated, political corruption became rampant. Samuel J. Tilden won a national reputation in 1871 for prosecuting the Tweed Ring of New York City, headed by William Marcy “Boss” Tweed, but Tammany soon recovered much of its prestige and influence as the Democratic city organization. The Republican party also had bosses, notably Roscoe Conkling and Thomas Collier Platt, and the split between Democratic New York City and Republican upstate widened. New Yorkers Chester A. Arthur (1881–85) and Grover Cleveland (1885–89, 1893–97) served as Presidents of the United States in the late 19th cent.

After 1880 the inpouring of immigrants from Ireland, Italy, and Eastern Europe brought workers for the old industries, which were expanding, and for the new ones, including the electrical and chemical industries, which were being established. Labor conditions worsened but were challenged by the growing labor movement, whose targets included sweatshops (particularly notorious in New York City). Muckrakers were particularly vociferous in New York in the late 19th and early 20th cent. Service as New York City’s police commissioner and then as a reform-oriented governor of the state helped Theodore Roosevelt establish the national reputation that sent him to the vice presidency and then to the White House (1901–9). A fire in 1911 at the Triangle Waist Company in Manhattan that killed 146 workers resulted in the passage of early health, fire safety, and labor laws including the Widowed Mothers Pension Act.