Never understood that one.
That's a subject that we've kicked around frequently here on FR, but part of the answer is that many of these Jewish 'Rats don't give much of a hoot about Israel.
The RJC, though, has been around since the Reagan era and is steadily growing in membership. We foresee a gradual movement of Jews away from the 'Rats, accelerated by the growing unpopularity of "Osama" Obama.
It really isnt that difficult. Most of the Jews came to the USA in the late 1800's early 1900's. Why? Because after a thousand years of persecution and pogroms of Europe, they decided to try the USA. Thousands and in some cases hundred's of thousands were slaughtered in these pogroms. (As in the case of the Cossacks)
In the 1800's a new "enlightened" form of Judaism arose. Reform Judaism. The majority of these new immigrants embraced this new form of Judaism, in part, IMHO, to avoid being persecuted by their new hosts. Reform Judaism does not require the strict observance of the Mitzvot (commandments), so they started like their hosts to eat pork and shellfish, not observe the sabbath and began intermarriage.
All the while, still stressing social justice. So, like any liberal religion, the government became the the new arbiter of justice, not G-d. Look at the World Council of Churches. Same thing. Anytime G-d is pushed out the vacuum is replaced by man-humanism. When G-d is replaced by government, freedom is replaced by tyranny. The Jew does have a powerful mind which can be used for great good or great evil. The Jewish soul needs the guidance of Torah and the obedience to HaShem to bring about true tikkun olam, repair of the world.
That is why they vote democrat. Previous persecution, liberalized religion to "fit in" and not be so "different" and thereby replacing G-d with government to bring about "social justice".