As the candidate who is both furthest left in the Democratic presidential field and least favorably-inclined toward Israel in his public statements, it would not inherently be surprising that Bernie Sanders has attracted the support of far-left political figures with a history of antisemitic comments and actions, including Linda Sarsour, Ilhan Omar, and Amer Zahr. It might seem surprising, however, because Sanders is Jewish, and one might think that (a) people with a history of antisemitic comments and actions are likely antisemitic; and (b) antisemites wouldn't endorse a Jewish candidate. Indeed, supporters of these Bernie endorsers have been quick to use their endorsements as evidence that they aren't antisemitic; after all, no antisemite would endorse a Jew for president. Right?
Wrong. The problem with this reasoning, and much of the discourse around antisemitism in general and on the far left in particular, is what one might call "the Nazi standard." In other words, to only recognize antisemitism when it resembles the most virulent, murderous version of antisemitism, that of the Nazis, a version that is outspoken and proud of its antisemitism, and considers Jews subhuman, beyond redemption, and marked for extinction.
But none of those things are necessary for antisemitism to exist. I try when discussing antisemitism (or other forms of racism) to distinguish it from "mere" prejudice. Someone might think that Jews are disproportionately cheap, or especially prone to being good at making money, or "clannish," without having any significant internal hostility to Jews, the way someone might believe that the Irish are prone to be drunkards, the Scots cheap, the Poles dull, and so forth, without evincing any significant hostility to them. It seems to me, though, that one or both of the following two things make someone antisemitic, and not just prejudiced: (1) one believes in anti-Jewish conspiracy theory, e.g., that Jews run the banks, control the government, etc.; or (2) one affirmatively wishes harm to Jewish people in general, though one almost always makes exceptions for "good" Jewish people (even leading German Nazis often had a favored Jewish acquaintance for whom they arranged an exit visa before the Holocaust began). You don't need to publicly acknowledge your antisemitism, nor, like the Nazis, consider Jews subhuman, beyond redemption, and marked for extinction.
Given those definitions of antisemitism, there is no particular reason that an antisemite couldn't support a Jewish candidate for president. If one believes in anti-Jewish conspiracy theory, for example that many or most of the world's Jews plot to control world governments to benefit Israel at the expense of their home countries, one can still believe that Sanders has shown himself to be an exception, that despite being Jewish he is not controlled (unlike many Gentile politicians!) by the "Jewish lobby." And if one wishes harm to the Jewish people, one can make an exception for a "good Jew" like Sanders–especially if one believes he is more likely to create or allow harm to other Jewish people than his presidential rivals.
Sure, a literal Nazi almost certainly wouldn't support someone of ethnic Jewish heritage, much less a self-identified Jew, for president under any circumstances. But literal Nazis are only a small fraction of the world's antisemites. One can, I assume, imagine circumstances under which some racists would support a black candidate over a white candidate, a misogynist a woman over a man, or a homophobe a homosexual over a heterosexual–especially if they endorsed policies that seemed more harmful to blacks, women, or homosexuals, respectively compared to their rivals. Similarly, once we recognize that one doesn't have to be a Nazi to be an antisemite, it's perhaps ironic, but not otherwise remarkable, that antisemites would support a Jew.
I should also note that a friend of mine has proposed a more cynical explanation for why Sarsour et al. have endorsed Bernie despite being antisemites. His theory is that they know he won't win, but since Bernie is both a mainstream candidate and a Jew, the fact that he is accepting their endorsement and indeed using some of them as campaign surrogates gives them both mainstream credibility and a megaphone for the course of the campaign, while hopefully blunting charges of antisemitism, at zero cost. I can't say this is definitely wrong, but it's not necessary to explain the endorsers' actions; even if they really, really want Bernie to be president and think he has a real chance, that doesn't conflict with the notion that they are antisemites.
David Bernstein is the University Professor and the Executive Director of the Liberty & Law Center at the Antonin Scalia Law School, George Mason University.