Jack MacDonald (nicknamed "Moscow Jack" Macdonald in the 1920s) born in Falkirk, Scotland, was a founding member of the Communist Party of Canada and one if its leaders. He was party Chairman from 1921 to 1923, and National Secretary from 1923 to 1929.

MacDonald supported the expulsion of Maurice Spector for Trotskyism in 1928. Subsequently, he tried to play a balancing role between the Tim Buck's Stalinist faction and the party majority headed by Finnish, Ukrainian and Jewish groups of which J.B. Salsberg was a notable figture. Macdonald failed and was expelled from the party in 1931 being accused of being a Lovestoneite (that is a supporter of Bukharin's Right Opposition). MacDonald, however, maintained that he was attempting to play an independent role. Accusations of "Lovestoneism" are further undermined by the fact that MacDonald went on to reconcile with Spector and joined the Toronto branch of the International Left Opposition (Trotskyist) Canada in 1932.

MacDonald and Spector sided with Martin Abern and Max Shachtman in a dispute within the Communist League of America that threatened to split the Trotskyist movement in North America in the early 1930s. The split emerged in the late 1930s, this time over the question of the class nature of the Soviet Union with both MacDonald and Spector siding with Shachtman in his split from the International in 1940. MacDonald subsequently dropped out of Marxist politics altogether.