Complex sentence: Thatcher was defeated by Hazen Argue in the 1957 federal election and again in 1958 but was nevertheless courted by the provincial Saskatchewan Liberal Party and became its leader in 1959 at the party's leadership convention by defeating three rivals.
Split into simpler sentences: Thatcher was defeated by Hazen Argue in the 1957 federal election and again in 1958. Despite his defeat at the federal level, Thatcher was courted by the provincial Saskatchewan Liberal Party and became its leader in 1959 at the party's leadership convention by defeating three rivals.