Complex sentence: Dawes believed that with the help of the McKinley Administration, he could win the Senate seat, however after McKinley's assassination, Dawes's hopes of winning faded because President Theodore Roosevelt preferred Dawes's opponent.
Split into simpler sentences: Dawes believed that with the help of the McKinley Administration, he could win the Senate seat. After McKinley's assassination, however, Dawes's hopes of winning faded because President Theodore Roosevelt preferred Dawes's opponent.