It promises to be close, brutally fought and with potentially significant repercussions for national politics.
The poll takes place on 5 May, in conjunction with votes for members of the London Assembly. While Khan and Goldsmith have been campaigning hard since they were selected as the Labour and Conservative candidates, it is only now that official nominations can be made.
The mayoral field will also include Caroline Pidgeon of the Lib Dems, the Greens’ Sian Berry, Peter Whittle of Ukip, as well as George Galloway, officially standing for Respect but with a website that barely mentions the party.
It is, however, shaping up as a straight two-way contest, with Khan the marginal favourite.
A recent YouGov poll put Khan on 32% of the vote, with Goldsmith at 25%, a margin that increased to nine percentage points for Khan when those who were undecided or said they would not vote were removed.
Labour is, however, taking nothing for granted in a contest which has, in its four previous incarnations, upset political orthodoxies. In 2000, Ken Livingstone, denied the Labour card, stood as an independent and took almost 58% of the votes, against 13% for the party’s chosen man, Frank Dobson.
Livingstone was eventually unseated in 2008 when Boris Johnson countered Labour’s perceived inbuilt dominance in inner London with a so-called doughnut strategy, maximising votes in the more suburban outer ring. Livingstone fought and lost to Johnson again in 2012, and so is absent from the poll for the first time.
The mayoral race has tended to focus heavily on candidates’ personal narratives, and here Goldsmith faces the possible disadvantage of being not just a second consecutive Eton-educated Tory hopeful but being even more privileged than Johnson, as the son of a billionaire tycoon.
Khan, in contrast, is the council flat-raised offspring of a bus driver father and seamstress mother who arrived in Britain from Pakistan.
In what is likely to be a bruising electoral battle, Labour will inevitably focus on Goldsmith’s background, while Khan has already faced coverage of alleged Islamist views within his extended family, offensive tweets from a now-removed aide, and a questionable Tory leaflet warning minority ethnic voters of Labour’s supposed “wealth tax on family jewellery”.
Goldsmith’s avowed ecological views had been seen as a possible lure for some left-leaning voters. However, he has proved lukewarm on the subject, making slightly confusing comments about getting rid of bus lanes and talking of digging up some cycle routes.
While turnout among the 5.7 million registered voters in London can be patchy, with fewer than 40% casting a ballot last time, the race will be closely watched outside the capital. It is arguably most crucial for Labour – if Khan does lose, and if the party achieves poor results in various local elections in England and Wales, that could put intense pressure on Jeremy Corbyn’s leadership.
Struggling to be heard amid the bipartisan noise will be Pidegon, a member of the Greater London Assembly; Berry, who won 3.2% of the vote for the Greens in the 2008 poll; and Whittle, formerly Ukip’s culture spokesman.
To stand for mayor, candidates have until 31 March to come up with at least 10 nominations from each of London’s 33 boroughs, as well as a £10,000 deposit, forfeited if they fail to get 5% of the first choices under the supplementary vote system.
Among those also hoping to be on the ballot paper are Lee Harris, the 79-year-old candidate for the Cannabis is Safer than Alcohol party and – still at the unlikely rumour stage – the singer Morrissey.
The mayor and the London Assembly have wide if limited powers, with the most influence exerted on transport, policing and housing.
Voters have until 18 April to register to vote. Details on how to register are on the official London Elects website.
This article was written by Peter Walker, for theguardian.com on Monday 21st March 2016 07.00 Europe/Londonguardian.co.uk © Guardian News and Media Limited 2010