There's 2 main sectors that control the support.
People of Jewish persuasion, and those who are favorably inclined towards Israel.
The first have AIPAC, which comes down like a bulldog on anyone even faintly criticizing Israel (they can go from 0-Godwin in .3 posts...
They work to make sure there's no overt criticism of Israel, and that whatever Israel wants or needs is procured by Congress. (There's other organizations, like the Anti Defamation League, but that's probably another post). Suffice it to say that both organizations can get the Jewish vote (or the threat thereof) to motivate any legislative slackers to toe the company line come election day.
The second include American's respect for, and rooting for the 'underdog'. As the closest thing to a 'Western' country in the region, they used a lot of that capital in the US to keep us supporting Israel, no matter what.
Finally, there is a small group of religious people who equate the establishment of Israel with the Apocolypse (and they're in favor of that...go figure), so if it causes the world to come to an end, well, it's all part of the plan.
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anahl nathrak, uth vas bethude, doth yel dyenvey...