Final answer:
Theodore Roosevelt gained the most from the Republican split in the presidential election of 1912.
Step-by-step explanation:
The candidate who gained the most from the Republican split in the presidential election of 1912 was Theodore Roosevelt. After being dissatisfied with the Republican party's nomination of William Howard Taft, Roosevelt formed his own party called the Progressive, or Bull Moose, party. Despite not winning the election, Roosevelt received more votes than Taft, indicating that he was able to attract a significant portion of the Republican voters.
Learn more about Presidential election of 1912