This is an old question that I have answered in many forums. There is a tradition of social justice within our religion. Jews were often the underdog and mistreated in most societies they lived in .There is a larger answer to the extreeme left radicals who are deviod of common sense and articulation.
The Jewish Community tends to turn to messianic figures in times of stress. Jesus and Shabetai Zvi are the most familiar of these figures. In this latter age messianism is no longer an option. The responses have been Chasisidism, Extreeme left marxist secularization and proto zionism. There is a loss of innocence that pevents religious Utopianism from being accepted. Man has achieved disasterous results when he aspires to become a god. The truth is that we need a set of checks and balances on power to control our bestial nature. Communism and Nazism are totalitarian attempts to reinvent man and create a new utopia. The problem is Utopia is impossible and man himself is flawed.
The harsh world of my grandparents gave birth to this horrible radicalism. Jews were not the only people who embraced Communism. Life was bad for everyone except Jews and Gypsies who were worse off as outsiders. My great granfathers life was typical.He lived in a forgotten village on the Bug river called Janover. He was drafted into the Czars army . The duration was 17 years with no vacations. Opressing ones own people was a side benefit. He took an axe and took off his trigger finger. This was apparently not original.
The reaction to this horible world of misery often had households where Zionists,Communists,Chasidic and regular folk . These divergent views would clash at a dinner table. Following emigration these arguments played out in America. The fact that my grandfather was from the wealthiest family in the poor village counted for little in the face of a pogrom. It meant that one was an outsider with a few extra possesions and diets.My grandfather served in the Polish army and was known for his marksman ship. He served and was still considered an outsider equaly likely to be beaten in a pogrom.
In America there was an identification with the outsiders blacks. People who lived as outsiders identified with outsiders in America. The unwritten part is that a great number of these people were Communists. They were motivated by a fantasy of a revolution of the masses. The revolution never came most moved on after one disaster of Communism followed by more. My Uncle was a firm Communisthe left Poland and went to the USSR. He sat out the Holocaust in Siberia. The experience cured him of Communism.
Most Jews gravitated towards the democratic party. When I registered in 1980 as a Republican it was considered a shocking development. Several people reminded me "but you are Jewish". I was a Cold War anti Communist and loved my country and the choice seemed logical.