Complex sentence: Earlier in the 20th century, divided government was rare, but since the 1970s, it has become increasingly common, mainly in part because of the Watergate scandal, which popularized the idea that a divided government is good for the country.
Split into simpler sentences: Earlier in the 20th century, divided government was rare, but since the 1970s it has become increasingly common. Mainly in part due to the Watergate scandal which has popularized the idea that a divided government is a beneficial for the country.