Complex sentence: The party gained significiant success in the elections and became the largest party of the opposition in 1958, but over the next decade the tide had once again turned and at the end of the 1960s, the party became instead the smallest party of the opposition.
Split into simpler sentences: The party gained significiant success in the elections of the 1950s but over the next decade, the tide had turned. At the end of the 1960s, the party had became the smallest party of the opposition.