The campaign for London mayor has gone national. Gordon Brown has sent in Tessa Jowell to revive Ken Livingstone’s reelection fight, and David Cameron has ordered every Tory MP to canvass for Boris Johnson.

Livingstone is trailing by 12 points in the polls and Downing Street has woken up to the damage the loss of London would inflict on Labour – possibly the last big test before a general election. With their decisions the two party leaders have tied their own fortunes to the outcome of the vote in six weeks’ time.

The upping of the stakes comes as it emerged that a second property developer behind a big skyscraper project gave Livingstone a campaign donation.

The Sellar Property Group, run by Irvine Sellar, gave a sum below the £5,000 declarable threshold in the run-up to the 2004 election.

Livingstone, who has spoken out repeatedly in favour of London having more tall buildings, backed the firm’s plan to build the controversial 66-storey Shard of Glass, which when completed will be the City’s tallest structure. A spokesman for Sellar said the group also gave money to the previous Tory candidate, Steve Norris.

Last week The Sunday Times revealed that Gerald Ronson, the developer behind the planned Heron Tower skyscraper in the City, gave Livingstone £4,990.

Meanwhile, the mayor, who won his first term in 2000 as an independent and had a tense relationship with Tony Blair, has submitted to curbs on his autonomy. He has surrendered powers to Jowell, with whom he worked on London’s successful bid to stage the 2012 Olympics.

Under the informal arrangement, she will mobilise Labour’s MPs and officials to campaign for Livingstone and will also play a part in fundraising.

Livingstone’s officials will seek Jowell’s approval for his spending plans but will retain several policies that the Tories say are divisive. They include the daily £25 congestion charge on so-called gas guzzlers, due to start in October.

Labour MPs in the capital are said by party insiders to have been “spooked” by a YouGov poll for The Sunday Times last weekend that indicated Labour has the backing of just 24% of the capital’s voters, compared with the Conservatives’ 49%.

A separate YouGov poll last week gave Johnson 49% of the vote against 37% for Livingstone. Brian Paddick, the Liberal Democrat candidate, had 12%. There are also candidates for a range of minority parties.

Transport for London, which is overseen by Livingstone, asked London cabbies to stop circulating leaflets backing Johnson’s campaign last week.

Three years ago Jowell was tasked by Blair with representing victims of the 7/7 London bombings, and Brown is said to believe that her soothing manner will help to counterbalance Livingstone’s abrasive style.

The mayor’s character has recently come under fire. Questions have been raised over his drinking – he said he drank whisky at mayor’s question time for “medicinal effects” – and the funding of community projects.

Lee Jasper, his race adviser, quit after the release of sexually charged e-mails to the female director of two groups that had received £165,000 from the London Development Agency (LDA), which is overseen by Livingstone.

The district auditor is investigating 13 projects funded by the LDA. Six are being looked into separately by the Metropolitan police.

The Tory leadership is also asserting its authority, ordering Johnson to tame his scruffy blond locks and tone down his flippant manner.

Since January, the Henley MP’s campaign has been run by Lynton Crosby, the Australian strategist who led Michael Howard’s 2005 unsuccessful general election campaign.

One activist said the “Back Boris” campaign is now being run so rigidly that workers must fill in time sheets listing their time spent electioneering each day.

“The mood has changed dramatically,” he said. “Before, it was: ‘We can win this’. But with the polls so strong, now it is felt we’ll look like incompetent idiots if we don’t win.”

Caroline Spelman, the Tory chairman, has written to all 194 MPs and prospective candidates telling them to spend five days campaigning for Johnson.

Some are grudging in their favours. One home counties MP said: “I wish Boris well but I don’t see why I should have to bang the drum for him.”

One official denied that Johnson, who once had to trek around Merseyside apologising for insulting Liverpudlians, is concentrating on securing a high turnout in the white suburbs.

Labour has yet to mobilise its MPs to campaign for Livingstone, who has numerous enemies in the party. An aide to the mayor played down the significance of Jowell’s role. “Ken is running this campaign, not her – but she is our point person for the national party.” Although crime and transport are expected to be the main themes of the campaign, Livingstone’s supporters are concerned that voters in the capital may register dissatisfaction over national issues.

David Smuts, a Labour member who sits on the party’s finance and industry group alongside Lord Kinnock and Margaret Beckett, has launched a website that allows members of the public to put themselves forward as independent candidates for the mayoral election.

Visitors to londonelectsyou.co.uk, which is being funded by a group of anonymous businessmen, can publish their profile and manifesto online. The candidate who gains the most votes will receive £50,000 to stand against Livingstone and Johnson on May 1.

The frontrunners include Samantha Day, an 18-year-old student from Bromley, who lists her dream candidate for mayor as Superman, and a poet called Stephen Micalef, who describes London’s bendy buses as “undignified Third World cattle trucks”.

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