London mayoral election 2016: The polling behind the Tories' latest attack ad on Sadiq Khan

The Conservatives want voters in London to always associate Labour's mayoral candidate with Jeremy Corbyn. The Tories' hopeful for City Hall, Zac Goldmsith, is behind in the opinion polls and the party has published its latest attack advertisement on Sadiq Khan in a bid to change the MP's ratings.

The "your choice for mayor in 100 days" poster has Khan on the left alongside Corbyn, with the caption "his man in City Hall". Goldsmith, however, is on the right and has "your man in City Hall" above his head. The advertisement comes after the Labour leader's approval ratings dipped to a disappointing -32, according to YouGov.

The Conservatives' reasoning is simple: associate Khan with Corbyn to put London's electorate off. The Tories also have some substance behind their attack since Khan backed the left-winger ahead of Labour's leadership election. Also, some of the London Underground (LU) unions behind the recent batch of Tube strikes – Unite and the TSSA – have made donations to Khan's campaign.

The Conservative candidate has also argued that the government's Trade Union Bill would limit walkouts on the Underground by enforcing a 50% strike ballot turnout. But so far, the Tories' attacks on Khan, a London MP and son of a bus driver, have failed to put Goldsmith ahead in the race for City Hall.

The latest opinion poll from YouGov for LBC radio, of more than 1,000 people between 4 and 6 January, predicted the Labour candidate would beat Goldsmith by 10 points (55% versus 45%, respectively) in the second round of voting come May.