Complex sentence: In 1966, Peabody successfully sought the Democratic nomination for the U.S. Senate seat for Massachusetts, which was open that year; he was defeated by a landslide in the general election by Edward Brooke.
Split into simpler sentences: In 1966, Peabody successfully sought the Democratic nomination for the U.S. Senate seat open that year from Massachusetts. However, he was defeated by a landslide in the general election by Edward Brooke.