It's true it is the crackpots around the edge of the left who are the worst, and they have little to no actual influence. But I have often seen clearly anti-Semitic conspiracy theorists not being challenged by people, when if they had made other sorts of racist comments they would have been. I've also seen very clumsy anti-Israel rhetoric not being challenged, when other language is carefully policed - and this is more mainstream.
But still, I find myself annoyed at joining in the soul-searching at this particular moment, given it's all been sparked by a bunch of anti-Corbyn partisans who can't even do any soul-searching because they don't fucking have any souls. But as killer b says, there's just enough truth for the attack to work. There were a fair number of people on my fb feed who last week thought that the best use of their energy was to defend what Ken Livingstone said. That doesn't look pretty, particularly when I know they'd come down like a ton of bricks on any clumsily worded sentiments about immigration or muslims.