Just 12 months ago, Labour had a Jewish leader. Today, we're told, it has a problem with antisemitism. How can this be?

It might be useful to distinguish between two forms of racism: verbal, and structural. Although the two often go together, they need not. For example, you'll hear far more racist language in financial firms than in the arts industry - but you'll also see far more ethnic minorities too. One business has more verbal racism, the other more structural racism. In this sense, Labour has a problem with verbal racism, but isn't obviously structurally racist.

Of course, even small amounts of verbal racism can create a climate in which minorities can feel excluded - and for that reason it must be ruthlessly eliminated.

It would, however, be complacent to pretend that Labour's problem is only with some borderline mentally-ill inadequates. There's a bigger problem than that, for two reasons.

First, there has always been a trend on the left to want to be "edgy" and "transgressive" - an urge that sometimes can lead to an underweighting of liberal Enlightenment virtues; it's in this sense that Nick has a point when he speaks of a "chronic condition" on the left. It's this, I suspect that has made Hamas and Hezbollah attractive to some lefties; the same urge led to support for the IRA in the 70s and - ironically - to posher lefties in the 60s spending their gap year on a kibbutz. It's this urge - amplified in some cases by a desire to appeal to the "Muslim community" - that has fuelled antisemitic language.

Secondly, even if antisemitism is very rare within the Labour party as Jamie Stern-Weiner says, it is a disproportionately big political problem.

Those rightists who are bleating about Labour's antisemitism are of course rank hypocrites: they are happy to use racism in the London mayoral election; laughed off the racism of Boris Johnson; and seemed relaxed about using an anti-Semitic dog-whistle in some attacks upon Ed Miliband. But this is only to be expected. The right will use any stick to beat the left. The left should not help by handing them those sticks. In tolerating even the slightest whiff of antisemitism, Labour is breaking one of the first rules of politics and of life: never give a cunt a chance.

In the last few days, we've seen a complete vindication of important leftist ideas. Capitalism is not just inefficient, but rapacious. And state institutions are not neutral public servants but corrupt partisans in a class war. But what has been the lead story on the BBC news this morning? Labour's antisemitism.

Yes, Labour has an antisemitism problem. But it has a bigger twat problem.