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.