For centuries, hatred of Jews was a straightforward occupation. It had diverse forms and many different motivations, some of them contradictory to one another, but the act itself was clear. One could say of Voltaire that his contribution to humanity outweighed his hatred of the Jews, and even - often - pretend not to notice that it was there, but no-one who took the time to notice could claim he didn't hate Jews. This was equally true for anyone who engaged in the practice from earliest Christianity (perhaps earlier) all the way to the mid-20th century. Most of the time the haters of the Jews were proud of their sentiments, and expected others to join them; offhand, I can't think of any examples of haters who pretended they were not hating (though an expert in the matter might be able to dig up a rare example).
Which is not to say that there weren't people who condemned the haters for their positions: of course there were. That's my point: both sides to those arguments accepted the premise they were arguing about.
Then came the Nazis, and gave antisemitism a bad name. Antisemitism, it transpired, could destroy the world. You want to stay away from it, and certainly not be tainted by association with it.
One interesting way of getting over this was the very strong academic tradition of claiming the Nazis weren't really all that antisemitc. Hitler was, of course, and Himmler, and Streicher, but most of the others didn't much care one way or the other, rather they had other agendas, or they got caught up in the excitement, or whatever. This interpretation was very convincing for a while, until a group of mostly German researchers effectively dismantled it in the 1990s. Yet that was mostly a discussion by the academics and their hangers-on the educators.
The second, more pervasive way of disassociating oneself from the taint of association with the Nazis was to redefine antisemitsm as only what the Nazis did: mass murder of Jews. That's awful. But lesser forms of prejudice, say, they weren't what the Nazis did and thus weren't antisemitism. This argumentation is still very much with us to this day.
The third disassociation was of course to love the Jews next door, but detest the anachronistic ones who engaged in such outlandish practices such as patriotism, wars of national interest, national particularism in any form, and so on. It's not antisemitism, it's anti-Zionism. See the Guardian. This form has a second strand, in which even the anti-Zionism is played down, with the pretense that one is merely critical of some of Israel's actions; this line of reasoning is the most sophisticated since Israel really does offer quite a bit to be critical about: so long as one is equally critical of everyone else, and recognizes the context, and makes the effort to understand what's really going on...
The upshot of all this is that people can engage in activity which for centuries would have been openly recognized as expressing animosity to the Jews, and the Jews and their allies must invest inordinate efforts merely to get to the starting line of rejecting the antisemitism.
Take this long speech by John Mearsheimer, which went online two days ago. It is being cited widely - Noah Pollak, David Bernstein, and of course, Mondoweiss and Juan Cole, to name just a few.
I'm not going to argue with Mearsheimer. The man's lack of access to the subject of internal Israeli politics and how they play out, and his willingness to tell falsehoods about Jews, is beyond embarrassing, though his ability to get away with it speaks volumes to the potency of Jew hatred in our time. The truly significant part of his speech comes about halfway through, when he begins to divide America's Jews into Righteous, Apartheid-Jews, and undecided. This is the ominous part: a man who is lying through his teeth about Israel, and totally disregarding the actions of the Palestinians, is classifying America's Jews and tarnishing them.
Anyone who cares about democracy and a free society in America needs to take note. The reason the antisemitism of the Nazis was so horrific for humanity was that it threatened decency everywhere. It's not yet back, Nazi antisemitism, but its forerunners seem to be.