Labour is portraying Boris Johnson as "high risk" and "a joke" as it launches a last-ditch blitz aimed at preventing him becoming Mayor of London next week.
With a knife-edge vote in prospect as Ken Livingstone tries to see off the biggest threat of his political life, Labour will flood the capital with 4.2 million "risk card" postcards highlighting the danger of electing the popular Tory MP and television celebrity. One says: "Don't vote for a joke. Vote for London." The common theme across the cards is that "suddenly he's not so funny".
In a back-handed compliment to Mr Johnson, Labour will call him "Boris" in its campaign literature, despite an edict that ministers should not refer to him by his first name alone in case it reinforces his appeal.
Labour will also appeal to supporters of Brian Paddick, the Liberal Democrat candidate, to use their second preference votes to support Mr Livingstone to "keep Boris out" in next Thursday's contest.
In an interview with The Independent, Tessa Jowell, the minister for London who heads Labour's campaign, said: "The risk of Johnson is going to be the focus of the campaign in the last week.
"Our job is to remind people just how much this election matters, and that Mayor of London is a big, complex and difficult job. This is not a job for somebody who is regarded as a clown. If he is Mayor next Friday, suddenly he is not so funny. People shouldn't risk that, they should vote Ken.
"London needs a serious candidate for a serious job. The dividing line is between a serious candidate who gets things done for London and the very risky choice is someone who is amiable but incompetent."
Although Mr Paddick is refusing to tell his backers how to cast their second vote, Ms Jowell said: "Brian Paddick is not going to win. So anyone who votes Liberal has to imagine what it would mean to wake up and find Boris Johnson as Mayor. Imagine Johnson being in charge of a £39bn transport budget and the £16bn Crossrail project."
She said Liberal Democrats should not make the Green Party's Sian Berry their second choice, arguing that would be a "wasted vote". If no candidate wins more than 50 per cent of the first preference votes, the top two – almost certainly Mr Livingstone and Mr Johnson – go into a run-off and the "second choice" votes they receive are added to their score. These are expected to prove crucial.
Ms Jowell said: "The Liberals and Labour have much in common in terms of their ambitions for London and Londoners – more public transport, greener public transport, a commitment to the environment, personally, city-wide and globally.
"It is about more than experience. It is about competence, vision, understanding London's challenges and responding to them. Ken is a big figure all around the world. People should remember that if they vote Ken out, they are getting rid of somebody pretty special who cares passionately about London."
Some Liberal Democrats have backed Mr Livingstone as their second choice. Alexis Rowell, a Camden councillor, said: "When you look at Johnson's shameful record and his appalling views, I really think Lib Dems have a duty to hold their noses and put Ken second on the ballot behind Brian Paddick."
But Vince Cable, the Liberal Democrats' deputy leader and MP for Twickenham, has rejected a plea by the Labour-affiliated Fabian Society for him to urge his party's supporters to make Mr Livingstone their second choice. He told the society: "I think you are mistaken in believing that I, or other MPs, can deliver Lib Dem votes to Ken Livingstone. I don't think our voters see themselves as a vote bank which can be steered in that way."