Complex sentence: He allowed the convention to make its own choice of running mate, and it selected Roosevelt, by then Governor of New York, a choice bitterly opposed by Senator Hanna, who nevertheless raised millions for the McKinley/Roosevelt ticket, which was elected.
Split into simpler sentences: He allowed the convention to make its own choice of running mate, and it selected Roosevelt, by then Governor of New York. That choice was bitterly opposed by Senator Hanna, who nevertheless raised millions for the McKinley/Roosevelt ticket, which was elected.