The election of 1912 split the Republican Party because of the ego and ambition of Theodore Roosevelt.
Theodore Roosevelt had become president in 1901 when President McKinley was assassinated. As McKinley’s vice president, Roosevelt became president. He won reelection in 1904, but chose not to run in 1908. In 1908, he picked William Howard Taft as his successor and had enough power in the party that it nominated Taft easily. During Taft’s presidency, however, two things happened.
First, Taft took a number of actions that bothered Roosevelt. Various issues arose (notably the issue of the tariff and of the Ballinger-Pinchot affair) that made Taft seem as if he was not as much of a progressive as Roosevelt was. These actions started to drive a wedge between the more progressive and the more conservative Republicans.
Second, Theodore Roosevelt essentially found that he was not ready to be out of politics. He liked being in the spotlight and he missed the importance of holding high office. When Taft’s actions made many of Roosevelt’s old supporters upset, it became more and more tempting for Roosevelt to run against Taft in the primaries.
Roosevelt ended up losing to Taft in the primaries. Then, driven by personal ambition and by his opposition to some of Taft’s policies, Roosevelt ran as a third party candidate. His Progressive, or “Bull Moose” Party, attracted many Republican votes. Taft and Roosevelt split the Republican vote, making it easy for the Democrat, Woodrow Wilson, to win.
By running first against Taft in the primaries, and then as a third party candidate, Roosevelt helped to split the Republican Party in this election.