For a politician who aspires to be a gentleman, London mayoral candidate Zac Goldsmith is the most awful coward. To use an old-fashioned word, he lacks the “manliness” to look an opponent in the eye. Rather than speak honestly, Goldsmith allows his PRs to whisper poison from behind the coward’s mask of anonymity.

When I call to ask for the details of the Goldsmith campaign’s accusation that his Labour opponent, Sadiq Khan, is a friend of Islamists, his press office told me that the Goldsmith campaign was alleging no such thing.

Well, that’s a relief. I can move on to my next story then?

Erm, not so fast, and no you can’t.

Goldsmith or, rather, his “people”, referred me to a speech by Michael Fallon. The Conservative defence secretary did indeed say that Khan could not be mayor of London because he had a long record of “speaking alongside extremists”. They pointed me to a piece I had missed in the Sunday Times. It showed how Khan had shared a platform with an Islamist called Suliman Gani, who had all the usual prejudices against women (they should be “subservient to men”) and gay sex (it is “unnatural”).

Goldsmith does not throw dirt himself, I learned. His hands stay clean and his deniability remains plausible. Instead, his propagandists make damn sure that journalists go to proxies in the Tory party and Tory press, who throw dirt on his behalf.

Khan ought to be able to duck it. Like many public figures on the left, he has been far too careless with his favours. But as someone who has played a small part in the bracing if not always enlightening rows among the ever-fractious comrades since 9/11, I can say that no one, not me nor anyone I know, ever thought that Khan was a misogynist or homophobe or antisemite or ally of the Muslim Brotherhood or apologist for Islamic State.

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He is what he seems to be: a working-class social democrat. After eight years of a do-nothing Tory mayor neglecting London’s problems, Khan’s willingness to think about its housing and pollution crises appears to be just what the capital needs.

To stop him winning what ought to be a shoo-in election, the obscenely wealthy Goldsmith is using religious prejudice to smear him. It wouldn’t work in normal circumstances, but it may do now; not because Goldsmith’s propagandists are even telling half truths about Khan but because they are telling the unvarnished truth about Jeremy Corbyn.

Labour’s leader is a “friend” of Hamas, Hezbollah and the IRA. He does not believe that the police have the right to kill Islamic State gunmen when they are murdering Britsh citizens. In the struggle between Russia and Europe, he is on Putin’s side on every issue that matters.

In the smearing of Khan, we are seeing the first of what will be many campaigns to bring down all associated with Corbyn’s Labour party with a new type of dirty trick. Usually, leaders are more moderate than their followers. They worry about attracting swing voters and holding on to the centre ground, and dissociate themselves from extremist supporters. Now in both Corbyn’s Labour and Trump’s Republican party, followers have to dissociate themselves from leaders so extreme that even their enemies can’t quite believe that they are in charge.

Imagine a paranoid American leftist – perhaps you don’t have to imagine him, perhaps you are him. For years, he has been saying that American conservatism is nothing more than a racket run by rich men, who win power by exploiting popular fears of Mexicans, Muslims, “the Other”. Then Trump starts winning and even he cannot believe how right he has turned out to be. Or imagine that you are a Louise Mensch type: an equally overexcited English rightwinger, who for years has also been ignored as you cried that Labour hated Britain and loved Britain’s enemies. Corbyn has proved you right, again to your astonishment and the astonishment of all who once dismissed you.

In these circumstances, the sensible strategy for mainstream politicians is to distance themselves from their leaders. Whatever their other faults, most Republicans running for office this year do not share Trump’s unwillingness to condemn the Ku Klux Klan. Khan, meanwhile, has already made it clear that he deplores Corbyn’s failure to sing the national anthem and warned him that he has come too close to comfort providing excuses for terror.

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But there are limits to how far from the fringe the mainstream can run. Politicians are greedy. They want the new followers the extremist leaders have brought into parties to campaign for them in London mayoral and US congressional campaigns. A gruesome debate under way among US senators makes my point for me. Republicans hope to finesse their position on Trump by somehow distancing themselves from the viciousness, while encouraging Trump supporters to vote for them.

Worse than the greed, however, is the complicity. A Trump or a Corbyn does not appear from nowhere and take over a party like the head of an invading army taking over a country. Men and women, who are not always extremists themselves, clear their way. As disillusioned American conservatives have said, Trump is just the absurd culmination of decades of Republicans denigrating government and branding all who oppose them as criminals and traitors.

The same applies to the Labour “moderates”. I could make much of the fact that Khan nominated Corbyn for the Labour leadership. No one thought that he wanted or expected him to win. It just seemed the “smart move” at the time because Khan wanted leftwing support for his mayoral bid.

It doesn’t look so smart now. I haven’t made much of Khan sharing a platform with Islamists, because I do not think there is much to make. But his willingness to do it reflected an overwhelming feeling in leftwing circles that it was racist to question those who supported the most intolerant versions of Islam. In other words, Tory mayoral candidates are not the only cowards on display. The failure of a generation of leftists to speak out meant that Corbyn could take the Labour leadership without even needing to make a fight of it.

Goldsmith is besmirching the reputation of an innocent man. But until moderate politicians find the courage to throw out the Trumps and the Corbyns, their opponents will damn their collaboration with the extremists who lead them. And if you say: “But that’s not fair”, they will reply: “No, but it is the new politics.”