Zac Goldsmith was the Tory even certain lefties had a guilty soft spot for. He was the apparently environment-friendly former editor of the Ecologist who threatened to` resign if the government approved a third runway at Heathrow. Principled, it seemed, and generally decent.

That Zac Goldsmith is no more, his reputation tattered by his own desperation for power. In his bid to become London’s mayor, Goldsmith is running a dirty campaign of smear, innuendo and divide-and-rule – and in doing so, he reveals the depths to which the Conservative party is willing to plunge to secure power.

Sure, Goldsmith has a disadvantage. London – and specifically much of its inner core – has increasingly become a Labour heartland. Labour may have been routed nationally last May, but the party led the Conservatives by 45 seats to 27 in the capital. Goldsmith has no control over his upbringing, but he is a privately educated multimillionaire in a city in which millions struggle with a housing crisis, and in which 44% hail from ethnic-minority backgrounds.

Zac Goldsmith’s fight to be London mayor: ‘I’m up against someone who poses a real danger’ Read more

His opponent Sadiq Khan is the Muslim son of a bus driver and a textile worker who grew up on a council estate. Goldsmith does not have the bumbling charisma of the do-nothing mayoral incumbent, Boris Johnson. But he has two possible routes out of his disadvantage. One is to run a campaign of hope and optimism that inspires people to vote for him. The other is to wage an ugly campaign to mobilise a good turnout in London’s suburbs for entirely negative reasons. He has opted for the latter.

One of the tools in Goldsmith’s desperate armoury is racial profiling. Letters signed by David Cameron himself have been specifically targeted at Londoners from Gujarati Hindu and Punjabi Sikh backgrounds. These letters feature the threat of terrorist attacks; crude allegations that Sadiq Khan wants to tax family jewellery (all tapping into stereotypes of the communities targeted); and Goldsmith’s welcoming of the Indian prime minister Narendra Modi. Modi, let’s not forget, failed to prevent a vicious anti-Muslim pogrom in 2002 when he was chief minister of Gujarat state.

Among the recipients of the letters was Barbara Patel: the letter presumed she was Hindu, when she is actually a woman of Jewish origins, married to a man whose family “are lapsed Muslims”. Her judgment is damning. Goldsmith was “attempting to cause division between the London Hindu community and its Muslim community. Shame on you.”

Patel isn’t a Tory – but Conservatives with the decency that Goldsmith lacks are speaking out. Shazia Awan stood as a Conservative parliamentary candidate and says she can’t see herself “backing Goldsmith after this disgrace”. She told ITV News this week: “Now the Conservative party talk a lot about ‘One Nation’, and that ‘we’re all in this together’, but to me, unfortunately, with the leaflets they are sending out for Zac Goldsmith at the moment, it seems like divide and rule.”

Then there’s Binita Mehta, the leader of Watford council’s Conservative group. “A blanket approach can seem stereotypical and patronising, and will certainly turn people off,” she wrote. “I hate to have to say this but obviously we ‘BMEs’ are much more sophisticated than these targeted letters suggest.”

Even worse, in a speech warning of the threat of terrorism – and highlighting the role of the mayor in combatting it – the defence secretary Michael Fallon slammed Khan as “a Labour lackey who speaks alongside extremists”. Let me be clear – this is a naked attempt to tap into anti-Muslim prejudices in a country in which “Muslim” and “extremist” are synonymous in the minds of all too many.

Still high from their surprise electoral victory, the Tories believe the politics of fear has a proven track record

Khan is a Muslim who voted for equal marriage – unlike many of Goldsmith’s Conservative colleagues – and faced death threats from extremists as a consequence. He risked his own safety to grant people like me civil rights that the likes of Fallon voted to deny. In Goldsmith’s own leaflets, Khan is described as “radical and divisive”. Again, you do not need sensitive hearing to detect the dog-whistle. When Khan’s campaign protested, Goldsmith hit back with an ugly phrase – claiming that his opponent was “playing the race card”.

In fact, Goldsmith is the man with questions to answer. He is to be investigated by the parliamentary standards commissioner. There are accusations that he failed to correctly declare in good time more than £120,000 from rich friends and relatives. The donors range from his mother to the wife of a convicted tax evader.

There are other questions, too. How would Goldsmith stand up for all Londoners when he has refused to be transparent about how much he benefited from his previous non-dom status? Or what about his use of a company based in the Cayman Islands tax haven to buy and sell his houses? And how can a man who was forced to resign from his local disability charity after voting to cut disability benefits claim to represent fairness?

Whether you like Sadiq Khan or not, there’s no question he has been running an upbeat, positive campaign focused on bread-and-butter issues such as the housing crisis and transport. That, surely, is how politics should be conducted in this country. But sadly, all too often, it is not. Still high from their surprise electoral victory last year, the Tories believe the politics of fear has a proven track record. If it succeeds in London, they will be emboldened to do it again.

That’s why I’m so depressed when I encounter left-leaning people who refuse to vote for Khan because he’s insufficiently radical. Not only would Khan’s defeat be a victory for the politics of fear and smear, but it would destabilise the Labour party. They may as well be voting for a leadership challenge to Jeremy Corbyn.

Khan may not be perfect, but his mayoralty would begin to tackle the capital’s growing social crises, not least housing. Goldsmith and the Tories must be taught a lesson. This time, the politics of fear must meet its nemesis.