It's not the numbers (though that could make a difference in a tight race, especially in New York or Florida) but the money. A person might be running for Congress in a district with a zero number of Jews, but if she is deemed "anti-Israel," Jewish organizations will generously supply her opponent with funds.
Why is everyone so concerned about where the Jewish vote goes?
It must be for reasons of moral validation.
I don't think so.
All groups have a right to support a candidate who supports their views.
For some reason, when Jews do this, the other side cites it and expresses outrage.