Not all Jews have been leftists, and not all are today, but it is not deniable that leftism has been very strong among European and American Jews for the last 100 years and more. I do view it as a tradition that has been handed down from generation to generation in a very large segment of Jewish society. Perhaps since during the last 30 years or so, Jews have been allowed more access to American society, many will abandon their left wing tradition, and move to a more traditionally "liberal" one. OTOH, other Americans seem to be giving up the old "liberal" tradtions with which the country was started.
I use the word cherish, and find it appropriate, because my view is that Jews hold to leftism in the belief that it will benefit humanity. Tikkun Olam, repair the world, as my rabbi used to say. FWIW, the top guy at the Hebrew School where my kids went was pretty much of a socialist, though I don't think they spoke of gentile blood or anything. The rabbi was an avowed leftist, but a good man who meant well in spite of that.
You would have to define Leftist.
Conservatives and Republicans until recent decades were not Jewish friendly.
Jews, like most, have a tendency to gravitate to where they feel comfortable. And for a long time that was the Democrat party.
NY, the most Jewish of States, had long term Republican Senators and Governors, because those individuals earned the trust of the Jewish community.
That Tikkun Olam crap is a very recent explanation. Jews moved Left because that is where they found emancipation from ingrained anti Semitism.
Jewish tradition- as can be seen by those who keep Jewish tradition, the Orthodox- is very Conservative.