Think of it as a test. Can you hold two apparently clashing thoughts in your mind at the same time? Or, put another way, can you condemn reprehensible words and deeds when they come from someone whose cause you otherwise believe is just?

The question arises after the Palestinian president, Mahmoud Abbas, declared in a rambling speech on Monday that the root cause of the Holocaust was not so much the Nazis’ genocidal hatred of Jews as the Jews’ own conduct, specifically their “social behaviour”, adding that he meant “their social function related to banks and interest”. Loosely translated, Abbas seemed to be suggesting that Jews brought the mass slaughter of six million upon themselves, thanks to their supposed stinginess, fondness for money-lending and for driving a hard bargain – to cite just a few of the hoary anti-Jewish stereotypes Abbas apparently had in mind.

On the face of it, those remarks are classically antisemitic, carrying an extra sting of victim-blaming for good measure. At a push, you could imagine someone justifying such a view by noting that tension between Jews and their neighbours in Europe was fuelled for centuries by antisemitic laws that banned Jews from owning land, excluded them from key professions and forced them to engage in financial activity religiously forbidden as “usury” to Christians. But Abbas didn’t say any of that.

Besides, the Palestinian leader has form in this area. In a January speech, he implied that European Jews preferred to endure the Holocaust than move to Palestine: “The Jews did not want to emigrate even with murder and slaughter,” he said. Even during the Holocaust, they did not emigrate.” Of course, the truth is that after 1939 the British mandatory government that then ruled Palestine barred all but a trickle of Jewish migration to Palestine – and the Jews of Nazi-occupied Europe were not exactly free to move where they chose.

What’s more, Abbas wrote a doctoral thesis at Moscow University several decades ago focusing on the Nazi period that drew on the writings of Holocaust deniers to question the number of Jews who were murdered.

In other words, when it comes to these latest remarks it’s hard to lend Abbas the benefit of the doubt. They are as awful as they look.

And yet, some will be wary of condemning them outright. They will worry that to do so is to side with Israel and its advocates, who have of course been quick to lambast Abbas. They will feel the urge either to justify or play down what the Palestinian leader has said or to look the other way.

But that would be to fail the test. For the right reaction is to condemn what Abbas said, safe in the knowledge that to do so does not in any way undermine the justice of the Palestinian demand for independence and statehood.

Witness the tweeted reaction of the journalist, Mehdi Hasan, a forceful advocate for Palestinian rights: “Mahmoud Abbas’s claim, as reported by the BBC, that Europe’s Jews were mass-murdered in the Holocaust because of their financial activities (!) and not because of Nazi anti-Semitism, is dumb, offensive, ahistorical and, yes, deeply, deeply anti-Semitic.”

Mahmoud Abbas's claim, as reported by the BBC, that Europe's Jews were mass-murdered in the Holocaust because of their financial activities (!) and not because of Nazi anti-Semitism, is dumb, offensive, ahistorical and, yes, deeply, deeply anti-Semitic. https://t.co/VISOMjCCHz — Mehdi Hasan (@mehdirhasan) May 2, 2018

That gets it right. Indeed, there are three conclusions one can draw from the Abbas speech, none of which should be unpalatable for supporters of the Palestinians.

The first is that yet another of the tragedies suffered by that people is to be poorly led. Abbas’ elected term of office expired nearly a decade ago, yet he remains in post. In that same speech on Monday, he threatened to tighten the squeeze on Palestinians in Hamas-ruled Gaza yet further, by reducing their share of the Palestinian Authority budget or cutting them off altogether – a leader adding to his people’s woes rather than reducing them.

The second is that Abbas is making the mistake of thinking the Palestinian case for statehood somehow depends on discrediting the Jewish claim. That’s why he seeks to minimise, or even blame the Jews for, the Holocaust – which, for many, underpins the moral case for a Jewish homeland – and why he remains attached to eccentric theories suggesting European Jews are not really Jews at all, or which otherwise deny the historic connection of Jews to Palestine. He fails to see that a two-state solution makes such arguments unnecessary: you can believe that both peoples, Jews and Palestinians, have a legitimate claim to the same land, which is why both have the right to a state of their own.

The third lesson of this episode stretches far beyond Israel-Palestine. There is a tendency in the current culture wars for partisans to behave as if their cause must be 100% just, while their opponents’ must be 100% unjust. It leads people to refuse to admit when their own side has got it wrong, as if that might undermine the wider case. It leaves supporters of Israel standing by Benjamin Netanyahu even when he engages in crude racism – warning that Arab citizens of Israel were heading to the polls “in droves” in the final hours of the 2015 campaign, for example – just as it can leave supporters of the Palestinians indulging Abbas.

It’s the same cast of mind that leads Republicans to cover for Donald Trump no matter how badly he behaves, that leads Tories to excuse the Windrush scandal, or Labour supporters to play down antisemitism within their own ranks. It’s a bad habit. If your cause is strong enough, you should have no fear of pointing out when it, or its leaders, err – and err badly.