Franklin Delano Roosevelt, commonly known by his initials FDR, was an American statesman and political leader who served as the 32nd President of the United States. A Democrat, he was elected four times and served from March 1933 to his death in April 1945. He was a central figure in world events during the mid-20th century, leading the United States during a time of worldwide economic depression and total war. A dominant leader of the Democratic Party, he built a New Deal Coalition that realigned American politics after 1932, as his New Deal domestic policies defined American liberalism for the middle third of the 20th century. With the bouncy popular song "Happy Days Are Here Again" as his campaign theme, FDR defeated incumbent Republican Herbert Hoover in November 1932, at the depth of the Great Depression. Energized by his personal victory over polio, FDR's persistent optimism and activism contributed to a renewal of the national spirit. Assisted by key aide Harry Hopkins, he worked closely with British Prime Minister Winston Churchill and Soviet leader Joseph Stalin in leading the Allies against Nazi Germany and Japan in World War II.