As to why the Jews back up the line were liberal, well, there's the question that needs to be answered. I don't know the answer, perhaps it has something to do with persecution in the old country. But I do know that nobobdy is going to wave a magic wand and change these traditions with mere logic about who supports Israel the most. The most liberal Jews I've known were fervent Israel supporters, to the point where they talked about going and serving in the military there if necessary. But that sure didn't have any effect on their politics here.
If they didn't feel that to some extent, I'd be worried. But running with a far-left crowd can change you. The leftier of those two friends I mentioned above once told me about a Jewish friend of hers who got some adjunct teaching job at Columbia ... the guy went in pro-Israel, and after about a year she found a Lebanese flag on his desk. I mean, what can you do for somebody like that.
Politically, support of Israel is a non-issue. Both parties vote overwhelmingly in support of Israel. Hence, there is little risk in terms of support for Israel regardless of what party you belong to.