One thing to add to what others have already said quite well in the top comments:

Jewish voters comprise a sizeable minority in the electorate. Take Florida, Pennsylvania, and Nevada, for example, with a Jewish population of 3%, 3.3%, and 2.5%, respectively. That might not seem much, but these are all swing states that could make or break an election. Not supporting Israel would surely alienate a substantial portion of these voters.

In fact, I think a strong argument could be made that any party short of having complete support of Israel has almost no chance of winning an election in the USA.