Support for Spains Republicans and Communists, as well as for either Hitler or Stalin, was often a partisan litmus test in pre-war America and, to a lesser extent, Britain (the Duke of Windsor ran afoul of the same problem as Lindbergh). I think Stalins formal alliances with both the Axis and Allied powers speak volumes about his ultimate loyalty, but I also think the Allies repeatedly made a grave error in choosing sides between far right fascism and far left communism, because while they are each others ideological antitheses, in practice they are indistinguishable. That's a failing of extremism, not of the right or left, both of which are vulnerable to it, and the conceit that your preferred faction is above it endangers you along with everyone else.