Complex sentence: He was first elected to the provincial legislature in a 1916 by-election and in 1919 he became leader of the Ontario Liberal Party but resigned as leader in 1921 due to opposition within his caucus to his prohibitionist stance.
Split into simpler sentences: He was first elected to the provincial legislature in a 1916 by-election and in 1919 he became leader of the Ontario Liberal Party. He resigned as leader in 1921 due to opposition within his caucus to his prohibitionist stance.