In the US and Britain, there is a campaign to smear anybody who tries to describe the plight of the Palestinian people. It is an attempt to intimidate and silence – and to a large degree, it works. There is nobody these self-appointed spokesmen for Israel will not attack as anti-Jewish: liberal Jews, rabbis, even Holocaust survivors.

My own case isn't especially important, but it illustrates how the wider process of intimidation works. I have worked undercover at both the Finsbury Park mosque and among neo-Nazi Holocaust deniers to expose the Jew-hatred there; when I went on the Islam Channel to challenge the anti-Semitism of Islamists, I received a rash of death threats calling me "a Jew-lover", "a Zionist-homo pig" and more.

Ah, but wait. I have also reported from Gaza and the West Bank. Last week, I wrote an article that described how untreated sewage was being pumped from illegal Israeli settlements on to Palestinian land, contaminating their reservoirs. This isn't controversial. It has been documented by Friends of the Earth, and I have seen it with my own eyes.

The response? There was little attempt to dispute the facts I offered. Instead, some of the most high profile "pro-Israel" writers and media monitoring groups – including Honest Reporting and Camera – said I an anti-Jewish bigot akin to Joseph Goebbels and Mahmoud Ahmadinejadh, while Melanie Phillips even linked the stabbing of two Jewish people in North London to articles like mine. Vast numbers of e-mails came flooding in calling for me to be sacked.

Any attempt to describe accurately the situation for Palestinians is met like this. If you recount the pumping of sewage onto Palestinian land, "Honest Reporting" claims you are reviving the anti-Semitic myth of Jews "poisoning the wells." If you interview a woman whose baby died in 2002 because she was detained – in labour – by Israeli soldiers at a checkpoint within the West Bank, "Honest Reporting" will say you didn't explain "the real cause": the election of Hamas in, um, 2006. And on, and on.

The former editor of Israel's leading newspaper, Ha'aretz, David Landau, calls the behaviour of these groups "nascent McCarthyism". Those responsible hold extreme positions of their own that place them way to the right of most Israelis. Alan Dershowitz and Melanie Phillips are two of the most prominent figures sent in to attack anyone who disagrees with the Israeli right. Dershowitz is a lawyer, Harvard professor and author of The Case For Israel. He sees ethnic cleansing as a trifling matter, writing: "Political solutions often require the movement of people, and such movement is not always voluntary ... It is a fifth-rate issue analogous in many respects to some massive urban renewal." If a prominent American figure takes a position on Israel to the left of this, Dershowitz often takes to the airwaves to call them anti-Semites and bigots.

The journalist Melanie Phillips performs a similar role in Britain. Last year a group called Independent Jewish Voices was established with this mission statement: "Palestinians and Israelis alike have the right to peace and security." Jews including Mike Leigh, Stephen Fry and Rabbi David Goldberg joined. Phillips swiftly dubbed them "Jews For Genocide", and said they "encourage" the "killers" of Jews. Where does this come from? She says the Palestinians are an "artificial" people who can be collectively punished because they are "a terrorist population". She believes that while "individual Palestinians may deserve compassion, their cause amounts to Holocaust denial as a national project". Honest Reporting quotes Phillips as a model of reliable reporting.

These individuals spray accusations of anti-Semitism so liberally that by their standards, a majority of Jewish Israelis have anti-Semitic tendencies. Dershowitz said Jimmy Carter's decision to speak to the elected Hamas government "border[ed] on anti-Semitism." A Ha'aretz poll last month found that 64 per cent of Israelis want their government to do just that.

As US President, Jimmy Carter showed his commitment to Israel by giving it more aid than anywhere else and brokering the only peace deal with an Arab regime the country has ever enjoyed. He also wants to see a safe and secure Palestine alongside it – so last year he wrote a book called Palestine: Peace Not Apartheid. It is a bland and factual canter through the major human rights reports. There is nothing there you can't read in the mainstream Israeli press every day. Carter's comparison of life on the West Bank (not within Israel) to Apartheid South Africa is not new. The West Bank is ruled in the interests of a small Jewish minority; it is bisected by roads for the Jewish settlers from which Palestinians are banned. The Israeli human rights group B'tselem says this "bears striking similarities to the racist Apartheid regime". Yet for repeating these facts in the US, Carter has widely called "a racist". Several universities have even refused to let the ex-President speak to their students.

These campus battles often succeed. Norman Finkelstein is a political scientist in the US whose parents were both Jewish survivors of the Warsaw ghetto and the Nazi concentration camps. They lost every blood relative. He made his reputation exposing a hoax called From Time Immemorial by Joan Peters which claimed that Palestine was virtually empty when Zionist settlers arrived, and the people claiming to be Palestinians were mostly impostors who had come from local areas to cash in. Finkelstein showed it to be scarred by falsified figures and gross misreading of sources. From that moment on, he was smeared as an anti-Semite by those who had lauded the book. But it was when Finkelstein revealed two years ago that Alan Dershowitz had, without acknowledgement, drawn wholesale from Peters' hoax for his book The Case For Israel, that the worst began. Dershowitz campaigned to make sure Finkelstein was denied tenure at his university. He even claimed that Finkelstein's mother – who made it through Maidenek and two slave-labour camps – had collaborated with the Nazis. The campaign worked. Finkelstein was let go by De Paul University, simply for speaking the truth.

Are the likes of Dershowitz and Phillips and Honest Reporting becoming more shrill because they can sense they are losing the argument? Liberal Jews – the majority – are now setting up rivals to the hard-right organisations they work with, because they believe this campaign of demonisation is damaging us all. It damages the Palestinians, because it prevents honest discussion of their plight. It damages the Israelis, because it pushes them further down an aggressive and futile path. And it damages diaspora Jews, because it makes real anti-Semitism harder to deal with.

We need to look the witch-hunters in the eye and say, as Joseph Welch said to Joe McCarthy himself: "You've done enough. Have you no sense of decency, sir, at long last? H ave you left no sense of decency?"

j.hari@independent.co.uk