LISA TERRY, a Conservative Party candidate in the London suburb of Croydon, knocks on a door. Behind it, an Asian family is unpacking cardboard boxes. They have just moved out of the city centre but are firm Tory supporters, they assure her. Sadly for Ms Terry, they may not stay long. Elsewhere in the street, new windows are being fitted. The sounds of drills and saws emanate from upstairs windows and back gardens. People here move in, move up and move out—usually to yet leafier places farther from the city centre. Outer London will feature prominently in the electoral battles of the next 12 months. Local votes are being held across England on May 22nd (coinciding with the European Parliament election). But only in the capital and in four other places will every seat be in play. The main parties are concentrating their energies on London’s fringes. These contain five of the opposition Labour Party’s six selected “battleground boroughs” and nine of the seats it is targeting in parliamentary elections to be held next year. Ed Miliband, Labour’s leader, launched the party’s national campaign in Croydon on May 6th. The following weekend Boris Johnson, the Tory mayor of London, was due to visit.

An electoral map (see above) hardly suggests the capital is a battleground. Apart from a Conservative slice through affluent western parts of central London and a dusting of Liberal Democrat yellow, particularly in the south-west, the politics of the city seems straightforward and settled: a doughnut with a bright red centre and a thick blue crust. But that crust is less thick than it was, giving Labour hope.

Demographic shifts are making the suburbs competitive. First-generation immigrants tend to cluster in the inner-city, but their offspring are moving into more middle-class neighbourhoods. Because Labour does better than the Conservatives among ethnic-minority voters, that helps Mr Miliband’s lot. And as central London has become pricier, other Labour-leaning groups—working-class folk and young graduates—are also heading out. Some may eventually pick up the politics of their Tory neighbours (see article), but so far the shifts are benefiting Labour more than the Conservative Party.

According to Tony Travers, an authority on local government at the London School of Economics, in the 1980s Labour’s local-election results were, on average, ten points lower in Croydon than citywide. In the 2000s, by contrast, they were slightly higher. A similar pattern holds in outer boroughs like Redbridge and Barnet, both now in the party’s sights.

Contesting such seats requires intense campaigning. Churning populations make it harder to keep track of voters. Blocks of flats guarded by entryphones—more common than in less densely populated areas—are harder to canvass. And residents commute into central London for work and entertainment so are often out. Good online communications and community organising are the answer, reckons Sarah Sackman, Labour’s parliamentary candidate in Finchley and Golders Green, a marginal seat in Barnet. Competition is stiffer than elsewhere in the country, too. By common consensus, local parties in London have more and younger activists—which, argues Gavin Barwell, the MP for Croydon Central, leads to more active campaigns.

As the Labour threat in outer London grows, Conservatives are preparing to deploy their party’s not-so-secret weapon. In Mr Johnson it has a deft and well-known campaigner who is not just popular among traditional Tory voters in places like Croydon but also has a knack for attracting new voters to the Conservative fold. If only he could be cloned.