'U f--ked up kamm. Anti corbyn campaign seen as a uk jewish zionist movement against a popular national tide," ran a message on Twitter. "Corbyn refuses to be interviewed by Oliver Kamm", said another poster. "He's one of the prime movers in the Henry Jackson Society. Arch Zionist. F--k him & the JC."

There's been a lot of this stuff lately and I don't wish to overstate its significance. The internet gives every malcontent a megaphone. But these are strange times. A major party is transforming itself utterly, in a way that deeply concerns British Jewry.

Jeremy Corbyn, the front-runner in Labour's leadership election, approached the JC last month to offer an interview. The editor asked me to do it. On learning I'd be his interrogator, Mr Corbyn pulled out.

I can only guess his motivations. I suspect he wished to convey warm words about community relations, and then realised that the way I do interviews is to inquire rather than take dictation.

Contrary to the pro-Corbyn tweeter, I have no connection with the Henry Jackson Society, a foreign-policy pressure group, and - as an outsider, having visited Israel only a handful of times - I've never adopted the honourable term, "Zionist".

The left now tolerates bigotry and embraces terror

But I'm a friend of Israel and I'm disturbed by Mr Corbyn's associations with extremists and his description of Hamas and Hizbollah as his "friends". As Mr Corbyn is uncommunicative, I can only draw inferences from his record and reticence.

I've debated once with him, on diplomacy towards Iran. It's a legitimate if arguable stance to advocate unconditional negotiation with aggressive states and terrorist organisations. My worry is that Mr Corbyn either doesn't recognise, or is unfazed by, the malevolence of such people.

What other explanation is there for his defence of Stephen Sizer, an Anglican vicar disciplined by church authorities for spreading the antisemitic conspiracy theory that Israel was responsible for 9/11? Mr Corbyn wrote: "Reverend Stephen Sizer seems to have come under attack by certain individuals intent on discrediting the excellent work that Stephen does in highlighting the injustices of the Palestinian-Israeli situation." It's a looking-glass world where an insanitary crackpot is praised for peacemaking. Yet, as writers on the left including the late Christopher Hitchens, Nick Cohen and me have warned, it's been going on for years.

It's no longer possible to assume that a declared progressive will defend free speech, secularism, women's rights, homosexual equality, cosmopolitanism and the spread of scientific inquiry.

Those are the values that cause me to admire Israel - a nation whose pluralist ethos will be fulfilled when there is an eventual two-state solution with a sovereign Palestine.

Yet now we have a left that tolerates bigotry, allies with theocratic reaction, embraces terror groups and espouses irrationalism.

Labour has long associations with British Jewry and Israel. Many exemplary MPs uphold that tradition. It is grave enough that Mr Corbyn's campaign and candidature stand outside it, yet the signs are that it's born of ideological conviction. It's not coincidental that, pressed in a BBC interview to condemn IRA terrorism, Mr Corbyn five times evaded the question.

Very well, then. If Labour is determined to abnegate its identity as a party of constitutional politics, I know where I stand. It's with this community, whose centre-left voters the party apparently neither wants nor respects.