Complex sentence: In 1958, he broke with "Union des électeurs" founders Louis Even and Gilberte Côté-Mercier, and joining Social Credit, became uncontested leader of the "créditistes" in Quebec.
Split into simpler sentences: In 1958, he broke with "Union des électeurs" founders Louis Even and Gilberte Côté-Mercier, and join Social Credit. He became uncontested leader of the "créditistes" in Quebec.