Time and again we have been told that Zac Goldsmith, the Conservative MP who is his party’s candidate to be London mayor, is a man of principle who stoutly sticks to his honourable beliefs even if that means breaking rank with the Tory leadership in parliament. His campaign has, for months, been portraying his Labour rival Sadiq Khan as, by contrast, a “machine politician” ready to change his position on all sorts of tricky issues if he thinks it will help him win City Hall.

But recent events underline that Goldsmith’s character and values merit far closer inspection than they have received so far from the bulk of the media. For months, his campaign has been peddling the fiction that a Mayor Khan would be Jeremy Corbyn’s “man in London” in the hope of depicting the Labour candidate as the compliant puppet of his allegedly dangerous leftist party leader. Goldsmith surely knows that is untrue: it just isn’t how the mayoralty works. He and his team should be perfectly aware that Corbyn will have no control at all over Khan if he’s elected on 5 May. Yet they’ve been saying the opposite anyway.

There’s also been the matter of the campaign literature sent - and sometimes miss-sent - to Indian, Tamil and Sikh Londoners, tailored to suggest that Khan, a Muslim of Pakistani descent, would not “stand up” for them as mayor and making highly dubious claims about Khan’s supposed indifference to foreign policy issues that might concern them and also the, frankly surreal, suggestion that electing him would risk taxes being raised on their family jewellery.

Asked about why such an effort had been made to solicit support from these three non-Muslim South Asian London communities and no similar, bespoke material produced for other faith or cultural groups in the city, neither Goldsmith nor his campaign have provided a convincing answer. They could have simply pointed out that Conservatives have been making a little headway with Indian-descended and some other Asian voters of late, so it seemed worth making an extra effort to woo them. But even that justification has not been offered. In the absence of any half-decent answer to such questions the suspicion must remain that the literature in question was designed to stir dislike of Khan on the grounds that he is Muslim.

And now we have been treated to the sustained escalation of Tory claims that Khan cannot be trusted with London’s security because of what are usually termed past “links” with fellow Muslims who have expressed extreme views. With Goldsmith trailing in opinion polls all year and little obvious evidence that the “Corbyn’s man in London” fib is having much effect, the “principled” Tory candidate and his campaign, which is being run for him by the company of negative strategy specialist Lynton Crosby, are now throwing everything they have at trying to destroy Khan by insinuating indulgent “links” to Islamist fanatics.

The escalation of this tactic began with a Goldsmith speech last Tuesday morning in south London, in which he accused Khan of “giving platforms, oxygen and even cover to those who seek to do our police and capital harm.” That evening came more of the same. In a debate with Khan held at the Institute of Directors, Goldsmith said that he did not believe that Khan himself held extreme views, but that past associations with people who did raised legitimate questions about his judgment.

Two names were prominently mentioned. One was that of Babar Ahmad, a resident of Khan’s Tooting constituency, who, after being locked up for eight years in Britain without being tried, pleaded guilty to a minor terrorism offence - basically, publishing two articles supportive of the Taliban on a website he ran from London - in the US following a long, high profile campaign to prevent his extradition. Khan had supported Ahmad and Goldsmith contended that this reflected badly on him. However, this attack line soon looked a bit untidy when it turned out that Goldsmith too had expressed sympathy for Ahmad’s case.

Also brought up by Goldsmith was Tooting imam Suliman Gani, who is reported to have expressed some highly reactionary views about women and gay people. Khan is accused of having repeatedly “shared platforms” with Gani, whom Goldsmith described last Thursday as “one of the most repellent figures in this country.” Yet almost immediately a less convenient version of the imam emerged.

On the same day, Gani tweeted a photograph of himself and Goldsmith together, apparently taken last autumn outside a Conservative Muslim Forum meeting held in Tooting to which both he and Goldsmith, who had by then been elected the Tory mayoral candidate, had been invited. He also showed a photograph of himself and Battersea’s Tory MP Jane Ellison sharing a platform as part of the campaign for the release of Guantanamo detainee Shaker Aamer, a cause to which other Tory MPs subscribed.

More significantly, Gani also tweeted a photograph of himself taken with Dan Watkins, who was the Tory challenger to Khan in last May’s general election. Gani said he’d supported Watkins in what turned out to be his unsuccessful bid to unseat Khan. He has since gone further. Yesterday, LBC broadcast an interview with Gani in which he said he had encouraged local Muslims to canvass for Watkins. He also confirmed that he had long since fallen out with Khan over the latter’s support for equal marriage.

This swiftly followed prime minister David Cameron’s intervention in the mayoral contest yesterday, when in an attack upon Khan he alleged that Gani “supports IS” (Islamic State). Gani has denied this and challenged Cameron to repeat the allegation outside the House Commons, threatening to sue him if he does. Parliamentary correspondents have reported being unable to extract from Cameron’s office any evidence that Gani “supports IS.” Meanwhile, a picture of a poster advertising an event organised in Mitcham in January has been circulated on social media. The event was headlined The Evils of Isis and Gani was billed as one of the speakers. This hardly suggests that Gani is an Islamic State fan.

In view of what had already emerged about Gani’s connections with Tooting and other Tories, Cameron’s attack on the imam on Wednesday raises puzzling questions. Why did he launch his PMQ’s attack when it had already been reported in several places that Gani had been actively supporting the Conservative Party in Tooting against Khan both before and after the general election? Had he somehow been unaware of this? Had neither he nor Goldsmith’s campaign actually noticed? Or could it possibly be that Cameron knew perfectly well about Gani’s falling out with Khan and his support for Khan’s Tory opponents but decided to make the attack on Khan anyway in the belief that he would get away with it?

It’s all very perplexing. But it certainly puts a different slant on the issue of politicians’ judgment as it relates to the London mayor campaign. Whatever else Gani may believe that is disagreeable, he is owed an apology by Cameron unless he can make the “supports IS” allegation stand up.

And what is to be made of Goldsmith’s role all of this? What, indeed, is to be made of him and his mayoral campaign as a whole? The “Corbyn’s man in London” line is highly disingenuous. His team has failed to respond to a request for the outcome of an investigation it said it would conduct into a report that one of its canvassers had disparaged Khan as “the Muslim.”

Goldsmith has said he sees and approves every piece of literature put out in his name. Yet he has failed to explain why special material, some of it signed by Cameron, has been aimed at selected South Asian Londoners and not at other groups, including Muslims as a whole and Pakistani Londoners. Until he provides a convincing alternative explanation, suspicions that a nasty motive lies behind them remain.

What part, precisely, did Goldsmith’s campaign have in Cameron’s decision to make his onslaught on Khan at PMQs on Wednesday? Does Goldsmith really approve of what Cameron said? What does his poisonous, devious, dishonest campaign say about the “principled” Goldsmith’s judgment? Exactly what kind of man is he?