Theodore Roosevelt was the 26th president of the United States, as well as a historian and a writer. He served as president from 1901 to 1909. He served as a Republican for almost two full terms. In 1912, Roosevelt left the Republican convention and went on to form his own political party. His decision to switch parties was based on his dissatisfaction with William Howard Taft, his chosen successor. Roosevelt returned to the GOP after losing his bid for office in his third term.