Theodore Roosevelt ascended to the presidency with the death of Republican President William McKinley, who was shot by an anti-government assassin on the afternoon of September 6, 1901 while he was visiting the World's Fair in Buffalo, New York. Governor of New York at the time, Roosevelt did not run for president in 1900 but was chosen as vice president by McKinley, partly as a favor to rich New York interests who did not always agree with the reform-happy 42-year-old governor and wanted him out of state politics. Roosevelt was sworn in on September 14, becoming the youngest president in American history, a distinction he still holds.

Roosevelt's first campaign for the presidency was for reelection in 1904. Roosevelt centered his campaign around his foreign policy experience and his successes in breaking up the great corporate monopolies of the time. His primary challenger in the Republican party was Mark Hanna of Ohio, who argued that Roosevelt was too radical in his policies toward big business. Unfortunately, Hanna died in February of 1904, leaving Roosevelt uncontested in that year's primary. He went on to easily defeat the Democratic candidate, Judge Alton Parker, in the general election.

In 1908 Roosevelt lived up to an earlier pledge to not run for reelection. Even though it seemed to be an unwritten rule that presidents would only seek two terms, it wasn't until 1951 that the 22nd Amendment limited the presidency to two four-year terms. Instead, Roosevelt supported his Secretary of War, William Howard Taft. Taft easily won the election by more than 1.2 million votes over perennial Democratic candidate William Jennings Bryan.

Dissatisfied with the Taft administration, Roosevelt again campaigned for president in 1912, this time for the newly formed Progressive Party, nicknamed the "Bull Moose Party." Taft sought reelection for the Republicans. In a political cartoon of the time, Roosevelt and Taft are depicted as Siamese twins heading in opposite directions. Not surprisingly, Roosevelt and Taft split the traditional Republican vote, leaving the election to the Democrat Woodrow Wilson, who easily won the electoral college with 435 votes. Wilson would be only the second Democrat elected between 1860 and 1932.