Complex sentence: The 1958 result was helped by an alliance with Maurice Duplessis's formidable provincial electoral machine, but by the 1962 election Duplessis had died and his Union Nationale party was out of office and in disarray, and Diefenbaker's support in Quebec had evaporated.
Split into simpler sentences: The 1958 result was helped by an alliance with Maurice Duplessis's formidable provincial electoral machine. But by the 1962 election, Duplessis had died and his "Union Nationale" party was out of office and in disarray, and Diefenbaker's support in Quebec had evaporated.