When Zac Goldsmith entered British politics approximately 10 years ago, he brought something special with him. He seemed decent and non-partisan.

Zac is the son of a billionaire, the late Jimmy Goldsmith. Some people would have been spoilt, perhaps ruined, by such a pampered family background. Not Zac. He has overcome the drawback of being born into extreme wealth (which in some cases can be every bit as debilitating as extreme poverty).

I live in Chiswick in west London, just across the Thames from Zac Goldsmith’s Richmond constituency. I heard reassuring reports that he was an excellent constituency MP. For this reason, like many other Tories, I welcomed his emergence as Tory candidate for mayor of London and planned to vote for him.

Wild horses could not make me do so now. Goldsmith's campaign for mayor has become the most repulsive I have ever seen as a political reporter.

Only two other campaigns bear comparison, both before my time. One was the infamous Bermondsey by-election of 1983 when the Labour candidate Peter Tatchell was targeted on account of his homosexuality. "Which Queen will you vote for?" asked an anonymous leaflet sent round the constituency in the final week of the campaign.

The other was the Smethwick campaign in the 1964 General Election. Peter Griffiths stood as the Conservative candidate against the shadow foreign secretary Patrick Gordon-Walker. Griffiths used the campaign to make a statement about immigration. "If you want a nigger for a neighbour, vote Labour" was the Tory slogan.

It was a moment of abiding shame not just for the Conservative party but for Britain. The vile Tory tactic was successful and Gordon-Walker was defeated.

The Labour Prime Minister Harold Wilson, however, called for Griffiths to be treated as a "parliamentary leper". So, by and large, he was.

In time he was shunned even by his own Conservative party. Griffiths was still a sitting MP when I became a political reporter in 1992. I remember him being pointed out to me. He was striding through the lobby all alone, a disgusting bigot shunned by his fellow MPs.

The Tories played the politics of racial division in 1964. Though these tactics worked in the short term, in the medium to long term they did great damage. Smethwick was a major reason why Labour have seemed to so many people to be the party of decency in the 1960s and beyond.

Goldsmith could have fought his Labour opponent on the issues had he wanted. Transport and housing policy are high on the list for Londoners, and there are serious flaws in Khan’s prospectus in both areas.

However, Goldsmith has not done this. His campaign has concentrated on Khan’s religion, alleging that Khan is a friend and apologist for "extremists" and cannot therefore be trusted to run a great city like London.

These claims are absurd. Khan is a mainstream Labour politician who has dedicated his career to advocating centrist views. He voted for same-sex marriage, thus estranging socially conservative elements in London’s Muslim communities. He loudly opposes a boycott of Israeli goods. He is a strong opponent of anti-Semitism. He has campaigned constantly against reactionary and so-called "extremist" forces within the Muslim communities.

None of this is good enough for Goldsmith’s Tory campaign, which has pumped out a constant barrage of propaganda portraying Khan as the pawn of sinister Islamist forces. Very worryingly, this policy comes right from the very top.

Fifty years ago at Smethwick nobody seriously thought that Sir Alec Douglas Home, then the party leader, had authorised Griffiths’ racism. In 2016, David Cameron is enthusiastically playing the politics of religious division. Yesterday he used Prime Minister’s Questions to claim that Khan was close to Suliman Gani, a south London cleric who believes in female subservience to men and is opposed to homosexuality.

The Goldsmith campaign team is reported to have followed the prime minister’s intervention by circulating a dossier alleging Khan’s links to anti-Semites, hate preachers, convicted terrorists etc.

The lack of context is deeply troubling. Whatever the truth about Gani (who has called the prime minister’s language "defamation at the highest level") Cameron culpably failed to mention that Gani was imam at Khan’s local mosque.

Khan could hardly avoid meeting him from time to time, just as his Conservative backbenchers cannot avoid coming across a local vicar with dotty or unacceptable opinions. Anybody listening to the prime minister would have gained the impression that Khan was actively seeking out Gani as his disciple or sympathiser.

But this selective distortion has been the Tory tactic right from the start. Khan has dealt with the majority of the so-called extremists cited in Goldsmith campaign propaganda in his capacity as a human rights lawyer.

What is Goldsmith trying to say? That terrorism suspects should not have legal representation?

The intervention of defence secretary Michael Fallon was yet more troubling than Cameron’s. Fallon invoked national security to say that it would be risky to appoint Khan mayor. Fallon is entering very dangerous territory with his hint that because Khan is Muslim he would be a security risk.

I still believe that Goldsmith is a decent man. But he has been weak. He has allowed himself to be captured by a cynical and amoral political machine. If he is to retain his reputation he needs urgently to change direction.

His tactics at present are profoundly at odds with the British tradition. Goldsmith’s attempt to stir up sectarian division by turning on a single community is horribly reminiscent of Republican presidential candidate Donald Trump in the United States.

And consider this: if Goldsmith’s campaign succeeds it tells every single British Muslim that there is no role for them anywhere in the British democratic system.

I have voted Conservative all my life. I will be proudly voting for Khan on the first and the last ballot when we Londoners vote for our mayor in two weeks' time. I urge everyone, including Tories, to do the same. By voting for Goldsmith Londoners will send out a terrible message about democracy in modern Britain.

- Peter Oborne was British Press Awards Columnist of the Year 2013. He recently resigned as chief political columnist of the Daily Telegraph. His books include The Triumph of the Political Class; The Rise of Political Lying;and Why the West is Wrong about Nuclear Iran.

The views expressed in this article belong to the author and do not necessarily reflect the editorial policy of Middle East Eye.

Photo: British Member of Parliament Zac Goldsmith (Fliker: Policy Exchange).