My Mother was a working member of the ILGWU back in the 40s (not a piecard). That union had leadership that mainly came out of the Socialist party. The Communist Party bitterly opposed them, and did everything they could to screw them over. (See the song "The Cloakmaker's Union".)

Tactically, I can't see much resemblance between Communists and Socialists at all!   Communists and Nazis use the same tactics. But the Socialists are committed to the same "will of the people" principle as other democratic political parties. So why didn't EMacC just join the Labour Party? Because the Communists in the east bought his plays?