Boris Johnson, Ken Livingstone and Brian Paddick are battling for votes before the poll on May 3.
Tonight's clash will be the first broadcast debate between Mr Johnson and Mr Livingstone since their explosive row on tax in a radio studio that boiled over into a shouting match in a lift.
For years, Mr Johnson could not decide whether he wanted to be a top journalist, editing The Daily Telegraph, or a senior politician, even Prime Minister.
Now he is firmly established as a career politician and if he defeats Ken Livingstone again next month many Tory MPs believe he has a good chance of eventually succeeding David Cameron as party leader.
He has charm, he is funny and is popular with voters who would not otherwise vote Conservative.
He is well known to voters from his TV and radio appearances and like Ken Livingstone is known by his first name alone.
He has a brilliant tactician, Australian election strategist Lynton Crosby, running his campaign once again and David Cameron has campaigned hard for him, despite the obvious tension and rivalry between them.
At the same time, Boris has skilfully distanced himself from some coalition policies, defending the City against attacks on the bonus culture and promoting his "Boris Island" airport plan for the Thames Estuary.
He is gaffe-prone, he is an old Etonian former Bullingdon Club member like David Cameron and he is accused by his opponents of being a part-time mayor because he continues to write a Daily Telegraph column.
He is accused of few real achievements during his four years in City Hall, apart from London's bike hire scheme.
He has also been accused of choosing key lieutenants badly.
Many have had to quit in embarrassing circumstances. His opponents also claim he panders to Tory voters in the suburbs and ignores the concerns of inner city, ethnic and minority groups.
Critics also claim his response to the London riots, when he was on holiday abroad, was feeble.
Mr Johnson says he will cut waste at City Hall, freeing up £3.5bn for services, put £445 back in voters' pockets by freezing the mayoral share of council tax, create 200,000 jobs, extend his bike hire scheme and secure a better deal for London from No 10.
Now aged 66, Ken Livingstone made his name as the left-wing leader of the Greater London Authority, taking on the Thatcher government, then he languished on the back benches in the Commons before running against the official Labour candidate for mayor, Frank Dobson, in 2000 and beating him.
He won as Labour candidate in 2004, but remains a divisive figure in the party. If he loses this time, he will be finished.
Despite his controversial reputation as "Red Ken", Mr Livingstone is vastly experienced at running the capital over many years, as Greater London Council leader and later mayor.
Bringing in the congestion charge against fierce opposition took real courage and he stood up to Tony Blair and Gordon Brown on Tube and bus financing.
His left-wing credentials, anti-war stance and support for minority causes mean he will poll strongly among London's big non-white population.
Despite his differences with many Labour MPs, he also has the party machine campaigning for him.
After George Osborne's unpopular Budget, Labour are also on a roll in the opinion polls, two polls this week giving them leads of 11% and 9%.
His age, his divisiveness and his controversial tax affairs. Labour MPs report that many voters believe he has had his chance and that having lost four years ago should not be standing again.
He also alienates many moderate Labour supporters. Even he admitted this week that some Labour supporters say they will not vote for him.
And the tax onslaught against his arrangement to pay himself through a company has been hugely damaging.
David Cameron unleashed a relentless attack on his tax affairs at PMQs yesterday. We can expect plenty more of this from the Tories. He also makes offensive gaffes, berating a Jewish journalist and lamenting the killing of Osama bin Laden this week.
Labour believe Livingstone's plans to cut Tube and bus fares is a big vote winner. He is promising an overall cut in fares for Londoners of five percent in the autumn of 2012, no further fare rises at all in 2013 and from January 2014 and in subsequent years no fare rises above inflation. Opponents say that is unaffordable.
Mr Livingstone is also pledging to reverse what he claims are cuts in police numbers made by Boris Johnson.
The former Met commander is standing for the Lib Dems for a second time, after party chiefs urged him to run again to prevent a maverick bid by former MP Lembit Opik.
Openly gay, he has had a civil partnership. He is seen as a decent, honest candidate, but lacks the flair of his two rivals, who are charismatic and fluent media performers.
He has acquired a pair of thick black glasses since the last mayoral election, which probably give him more gravitas.
A much more serious character than his two jokey rivals, he is well respected on policing issues, speaking with the experience of a former commander in Brixton.
So he is likely to appeal to Londoners worried about crime and appalled by last year's riots.
He is also less likely to make embarrassing gaffes or cause offence than either of his two major rivals.
He will benefit from being on the platform in the Sky News debate and other high-profile hustings alongside the two front runners, just as Nick Clegg did during the 2010 general election. Listen out for: "I agree with Brian."
Alongside Johnson and Livingstone, Mr Paddick can appear dull and wooden. And although some politicians may not like it, this is a popularity contest.
Like his two main rivals, he stood last time and many voters will think the Lib Dems should have chosen a fresher, more flamboyant candidate this time, even Lembit Opik.
His vote got badly squeezed last time and the slump in the Lib Dems' poll ratings caused by the woes of being in coalition with the Tories may badly damage him.
Some pundits even predict that he could be pushed into fourth place by independent candidate Siobhan Benita.
His ideas on policing include making criminals repair the damage they do and ending racist stop and searches.
His transport plans include a one-hour, hop on-hop off bus ticket and "early bird fares" for people travelling before 7.30am. He also proposes the Tube should be shut for weeks for repairs until the job is done instead of routine weekend shutdowns.