That's been addressed here many times.
The brief answer is that the vast majority of my US co-religionists are descended from folks who fled violent anti-semitic attacks in central and east Europe and Russia around the turn of the last century. At that time, the only ideology that opposed the attacks (many of which had been instigated/approved by various churches and monarchs) was nascent secular socialism. It was that political system those fleeing took with them to our shores.
My family came to the US after Hitler's genocide, so I have a very very different view of "National Socialism".
Freepers need to read up on the massive and periodic pogroms in Russia and Poland etc. The Jews there turned to the left and socialism and communism as a protection response to the pogroms and other anti-Jew oppression. When those Jews immigrated here they tended to be leftists and socialists so joined the Democrat party. They wanted the best for humanity but with retrospect we can see this was (and is) the wrong way to go
My grandparents fled to avoid being conscripted into the Czar’s army. The royalty was roughly on the side of those who mounted pogroms