This piece by Jonathan Cook offers a straightforward defense of moral responsibility with respect to Israel (or, in principle, anything else): if you defend what it does, you share moral responsibility for what it does. Because “the West” collectively defends Israel so uncritically (particularly “Western” governments that give Israel both material and moral/political support), “the West” is complicit in Israel’s crimes in ways that it’s not complicit in other, equally unjust, or worse crimes. It’s legitimate in this context for citizens of “Western” governments to single Israel out for special criticism.
One legitimate motive for doing so is to avoid moral complicity in Israel’s crimes via support given to the governments that abet these crimes. There certainly are other, illegitimate motives as well, e.g., anti-Semitism. But the illegitimate motives don’t negate or cancel out the legitimate ones. Their existence just requires disavowal–once again, to avoid complicity in them. In fact, responsible partisans of the Palestinian cause have always disavowed anti-Semitism. It’s pro-Israel organizations, in North America as well as Europe (not to mention in Israel itself), that have served up endless rationalizations for Israel’s crimes, and endless defamations of its critics.
In any event, I’m gratified to see, and happy to circulate, this message that’s been making the rounds in pro-Palestinian quarters:

The message was posted on Facebook by a friend of mine in the West Bank whose family members have been imprisoned, who has himself been shot by rubber bullets, whose house has been violently broken into by the Israeli military, and who lives in a village, Tuqu’, that is physically surrounded on all sides by settlements and military stations, including one at the entrance to the village itself.

The Palestinian cause means too much to people like him—the stakes are too high—to allow it to be appropriated by anti-Semites.* White nationalists have explicitly decided to exploit the Palestinian cause for anti-Semitic ends, as have Muslim anti-Semites, and I’m sure, other kinds. But as the preceding message makes clear, such people are both irrelevant to, and at odds with, the rights-based motivation behind the Palestinian cause. We can’t merely “ask” that such people stay away from us. We have to do what it takes to make sure that they do.
*I slightly changed the wording of this after posting.
**There’s been a spate of recent articles reporting on a rise in anti-Semitic attacks on Jews in the US, UK, and elsewhere. To the extent that these really are attacks, whether verbal or physical, they have to be disavowed and condemned, but some of what’s being reported seems like undignified whining masquerading as victimhood. A passage from “Jewish American groups appeal to Biden White House after string of antisemitic attacks related to Israel” (Washington Post, May 21, 2021):
Joel Rubin, who handled Jewish outreach for Bernie Sanders’s 2020 presidential campaign, said the current level of vitriol in the United States against Jews is higher than he’s ever seen it. He pointed to an increased use of the term “apartheid” by progressive politicians and the appearance of White nationalists in the Palestinian cause.
Three comments:
(1) How is Joel Rubin singlehandedly in a position to judge the “vitriol level in the United States against Jews”? The claim presupposes that his intuitions are a kind of finely-tuned vitriolometer. They aren’t.
(2) How does “an increased use of the term ‘apartheid'” in reference to Israel indicate vitriol of a sort relevant to the expression of anti-Semitism? B’Tselem and Human Rights Watch (among many others) have argued that the term fits Israel’s policies. How are they wrong? They certainly can’t be anti-Semitic and right.
(3) And what does it mean to say that “White” nationalists (aka white nationalists) are appearing “in the Palestinian cause”? The Palestinian cause is not a room into which white nationalists have been granted entry, or a club to which they’ve acquired membership. The reality is that white nationalists are just cynically and ignorantly pretending to espouse the Palestinian cause for their own racist purposes, and will continue to do so no matter what the rest of us do. To speak of them as part of or “inside” the movement for Palestinian rights is willingly (and disingenuously) to fall into a trap that the white nationalists themselves have designed. It’s hard to believe that anyone can be that stupid, much less that stupid and once have been “in” the Sanders campaign. But there it is, in black and white.