People of Britain: congratulations are in order. You have now joined ferret owners, sidewalk artists, hot dog vendors, publicly funded attorneys for poor people, low-income community college students, museum curators, a couple of innocent black men shot dead by the police, the sections of the New York City charter governing rules of succession to the mayoralty and, of course, Hillary Clinton, as objects of Rudy Giuliani's demagoguery and wrath.

You may by now have heard the story. In a radio ad that his campaign prepared for New Hampshire voters, Giuliani tells listeners that he was diagnosed with prostate cancer in 2000 and goes on to say: "My chance of surviving cancer - and thank God I was cured of it - in the United States: 82%. My chances of surviving prostate cancer in England: only 44% under socialised medicine."

The numbers are false. The actual five-year survival rate in Britain is 74%, which is still lower than America's, but obviously high enough for the figure not to have constituted fodder for a campaign commercial. (Even the remaining, much smaller difference, is largely explained by more widespread screening in the US, which catches many more incidents of prostate cancer that are non-lethal).

It turned out that Giuliani's numbers were from a seven-year-old article in a conservative policy journal. The article was written by his own healthcare policy adviser, who admitted that his comparison was a "crude" interpretation of a study by a respected health policy group. The group, in turn, said the article's author had grossly misused its numbers.

That's about as red-handed as anyone in politics gets caught these days. But when asked if the campaign would continue to use the figure, a Giuliani spokeswoman said, "Yes, we will."

I know the form all too well. I covered Giuliani for a dozen years in New York (note to angry American rightwingers preparing to email me a warning to keep my foreign nose out of their business: I'm as American as a Ford F-150).

The man lies with staggering impunity. But here's the thing: he does it with such conviction and such seeming authority that people who are not inclined to study the matter will believe him - will in fact be utterly convinced that Giuliani is speaking the gospel truth, and they will prove almost impossible to shake from this conviction.

Giuliani's hypocrisy with regard to this ad doesn't end with the fake statistics. As Joe Conason noted on www.Salon.com, Giuliani was at the time of his treatment the mayor of New York and enrolled in a nonprofit health maintenance organisation for government employees - that is, mini-socialised medicine. And as Ezra Klein noted on Comment is free, the treatment that saved Giuliani was developed in Denmark - which, as Klein drolly notes, "is both in Europe and has a universal healthcare system".

But none of this will stop Giuliani. He will say and do anything he feels he needs to say and do to get power.

Newspapers write that he was "liberal" on social issues in his mayoral days, as if his positions on abortion and immigration were matters of conviction. Nonsense. He took the positions he needed to take to be elected in an overwhelmingly Democratic city. (Although to grant him a speck of humanity, I'd guess that his pro-gay rights views were more or less genuine: anyone living in the city gets to know many gay people.)

And now he is saying and doing whatever he needs to say and do to get millions of rightwing Americans to support him. He recently told a meeting of social conservatives that his reliance on God "is at the core of who I am". As mayor he was known to attend mass almost never, he obviously cheated serially on the wife (wife No 2) he married in the Catholic church, and the only occasions on which I can remember him invoking God when he was mayor were the two times he was forced to say "so help me God" in taking the oath of office.

But forward he will charge, telling more lies with even more impunity. And immunity, because in a culture where a sense of history is largely limited to remembering certain stirring television images, he will for the most part get away with it, confident in the knowledge that the main thing most Americans will ever recall about him is the film clip of him running from the rubble of the World Trade Centre on September 11. A far smaller percentage will know that the reason he had run was because he had catastrophically decided to place his emergency command centre in the tower complex - the only building in New York that had previously been the target of a major terrorist attack.

And by the way: shame on Gordon Brown for inviting him to No 10 in September. Yes, there's a long tradition of presidents and prime ministers welcoming party standard-bearers from across the pond. But Giuliani isn't yet that. Brown had no business giving him the kind of special benefit that an audience with a prime minister bestows.

Brown and all of Britain will be better off the sooner they figure this out: Giuliani is a dangerous man. George Bush with brains. Dick Cheney with better aim. Consider yourself warned.

· Michael Tomasky is the editor of Guardian America michael.tomasky@theguardian.com