President Taft sought a 2nd term, but he would not go unopposed. His predecessor Teddy Roosevelt came to take back his job.
Roosevelt easily defeated Taft in the democratically run primaries except the corrupted use of delegates gave the nomination to Taft. So really the nomination was stolen from Roosevelt.
Not feeling too happy, Teddy started the Progressive Party and his fellow wing of Republicans switched parties for the time being.
There was also a suprise nomination to New Jersey Governor Woodrow Wilson, who managed to get 2nd place until front-runner Champ Clark was shunned by many Democrats. After that it was an easy victory.
Eugene V. Debs returned to run with the Socialist Party again. By this time, the party had claimed over 1000 elected officials in 33 states. However they were basically broke to run a national campaign.
The election was so spilt that nobody could see the whole picture. Republicans threw their support behind Taft, while others left to join Roosevelt. Debs campaigned that the 3 other men were really just backed by corporations. The Socialist Party didn't manage to get the support of labor. Everyone else just supported Wilson.
So that's basically how it went. Wilson won by a landslide with 435 electoral votes. Roosevelt got 88 while Taft got only 8 little votes. Taft was given the worst defeat for any Republican candidate, and it threw a major party in 3rd place for the 1st time.