Complex sentence: Thompson refused to cede the leadership and the party split with most of the Quebec MPs following Caouette into his new "Ralliement créditiste" party in 1963.
Split into simpler sentences: Thompson refused to cede the leadership of the party to Caouette. This caused the party to split: most of the Quebec MPs followed Caouette into his new "Ralliement créditiste" party in 1963.