Andrew Boff, the former leader of the London Assembly’s Conservative group, said that he and many other Tories in the capital were “really troubled” by the Goldsmith tactic of painting Labour’s candidate as an extremist.
London’s results are due on Friday evening and a raft of polls have been giving Khan a double-digit lead that suggests an end to the Tories’ eight-year grip on City Hall under Boris Johnson.
After the polls closed on Thursday, Boff told Newsnight that he had told his colleague that it was “a mistake” to attack Khan with such allegations as it undermined the party’s hard work in appealing to the Muslim community.
“I don’t think it was dog whistle, because you can’t hear a dog whistle. Everybody could hear this,” he said.
“It was effectively saying that people of conservative religious views are not to be trusted and you shouldn’t share a platform with them and that’s outrageous.”
“I was really troubled by one particular aspect [of the campaign] and that’s when he started equating people with conservative religious views with sympathising with terrorism. That sent a message out to many of the communities in London that’s very difficult to justify.”
Boff's views are shared by many London Tory activists contacted by HuffPost UK, who fear that Downing Street and Conservative Party HQ's strategy has set them back years in the capital.
David Cameron has led the Tory charge against Khan, using Prime Minister’s Question time to claim that he was not fit to run London because he had shared a platform with Islamic ‘extremists’.
He repeated the claims this week even though it had emerged that one cleric he’d accused of supporting Islamic State, Sulayman Ghani, had been a Conservative supporter against Khan and had appeared on the steps of No.10.
Furious Labour MPs have claimed that it was “racist” to suggest that Khan’s Muslim faith was a danger to Londoners.
But despite claims from his family that he was uneasy with the campaign, Goldsmith has repeatedly defended his decision to target his opponent’s record of sharing platforms with people like Ghani when he was a human rights lawyer.
Referring to Goldsmith in particular, Boff said: “He received advice and he was wrong to accept advice [about the campaign]. This kind of politics I didn’t think was Zac.
“We chose to use this particular policy meme as the centre of the campaign, it was ridiculous…
"They seem to forget 2014 was not a great year for us in London. One of the few boroughs that swung to us was Newham where the Conservative there actively engaged with the Muslim community. Those bridges that have been built have been blown up.
“I’m not alone. We don’t shout about it during the campaign.
"I did bring it up during the campaign to Zac’s team I thought this was a mistake for the future integration of London. If you are a London politician this is just a bizarre thing to do.”
“I don’t want us to do this in London again, it’s done us real damage.”
Last weekend, the Tory mayoral candidate wrote a piece for the Mail on Sunday, illustrated with a photo of the 7/7 bombings and headlined “Are we really going to hand the world’s greatest city to a Labour party that thinks terrorists are its friends?”
Former Tory chairman Sayeeda Warsi tweeted “Are we Conservatives fighting to destroy Zac or fighting to win this election? In the real world, Londoners worry about housing, jobs and NHS. In the world of politics, new reality TV show ‘Britain’s Biggest Bigot’ launched!”
When asked by Parliament’s The House magazine last month if his tactics were ‘negative’, Goldsmith replied "so be it".
Some Tories believe that with a low turnout in the capital, he could still have a chance of snatching victory, but privately many have written off his chances.