I think a lot of Jews vote democrat because they mostly live in and around the major cities where the democrats have monopolized civic life. In other words, they're voting for democrats like they're loyal fans of the local team, and they don't critically think about the issues...like most people.
Yes, the political leanings of the neighboring non-Jewish populations do have some influence on Jewish voting patterns.
Big city Jews (excluding the Orthodox and the more recently arrived Jews from communist eastern Europe) will generally give a higher percentage of their vote to Democrats than Jews in suburban or exurban areas.