No argument about Horowitz: he is great, but sadly, a distinct minority. And, I don't know how or why anti-semitism compels someone to join the Communist party or fight for the Commies in Spain. It's fine to be for FDR...it is not fine to be a Red.
When Jewish political sympathies start to fall more in line with the rest of U.S. society I will finally believe that the pernicious old commies and their spawn have been won over.
Jews attracted to communism were attracted to theoretical communism, in my opinion. They thought it would be fairer and in a communist society Jews could finally fit in and be accepted by the larger society. They believed everyone would be treated the same and the terrible pogroms pumped up against them every Easter would end because the state would not support, promote, or encourage religion. They also believed the power of the state would protect them.
We see that in practice a communist state is even more oppressive and dangerous than an autocratic one.
You can't begin to understand the attraction until you, as a people, have lived for centuries in abject fear.
Paul Johnson does a fairly good job of explaining all this far better than I can in his histories.