The reason for the antisemitism differs depending on whether the antisemite is on the right or on the left.
Antisemites on the right: On the right, as Carl, Saxon, and other fundamental Christians show, it’s all religion-based. They smear and denigrate Jews living in the 21st century because a small little group of 1st century Jews were angry at Jesus, and might have influenced his crucifixion. Basically, 2,000 years ago, when there were about 5 million Jews, 0.0001% of them hated Jesus. The other 99.999% never heard of the guy.
I'm sure a lot more knew about Jesus at the time he lived (if he actually lived) or in the decades following. Even Josephus talks about Jesus and his followers, and that's about as contemporary as you get.
More to the point, the theological question is that Christians believe Jesus fulfilled prophecy and Jews deny he did. As an atheist, I find this argument to be a little silly. But if you think there's a magic man in the sky telling you what kind of sex to have or what kind of food to eat, there you are.
Antisemites on the left: Today, most of the antisemites are on the Left, screaming “Death to Jews,” bullying the Jewish kids, setting up “No Jew Zones,” etc., etc. The root of these antisemites is their resentment that a small little tribe, subjected to the worst persecution and bigotry for 2,000 years, manages to rise above to become above-average in achievement: Nobel prizes, educational attainment, income, whatever. It shows that with the right attributes and choices in life, racism or bigotry does NOT hold people back. IOW, it reveals the lie to the Left’s insistence that blacks are mired in poverty due to racism.
Or maybe they just resent the sense of entitlement.
You know, that it's okay for you to steal someone else's land.
Or oppress them. (Especially bad given what the Nazis did to them.)
You try to pretend that people on the left don't have a disagreement with you that is valid. I would say the opposite. If you are against racism, against colonialism, against the First World exploiting the Third World (liberal), then supporting Israel is ideologically impossible.
The real problem is Jews are now accepted as white people, and their first response has been, "Let's do all the awful stuff that White people do!"
It reminds me of when I was in college and much more conservative than I am now. Anyway, I was in the Student Government, and we had this smarmy liberal Jew who was trying to impress the black kids with how down with the struggle he was by oppossing Apartheid in South Africa. To which I asked him, "So how is that any different than what Israel does to the Palestinians?"
Man, you should have seen this guy go 'Humana-humana-humana'.