I think the problem is that Israel has become synonymous with Judaism, and Israel is currently in the grip of some pretty extreme right wing fascists. So, if you're anti-Israel it's being equated to antisemitism by those same politicians because one of the necessary pillars of fascism is a perceived external threat to keep the populace in line ("Only we can protect you from the enemy!" .. or as another wannabe dictator put it "Only I can keep America safe!")