THE number 4 bus starts its journey along the spine of Islington near the Whittington Hospital. Last month some 4,000 people met there to protest against proposals to close the hospital's accident and emergency department, and its maternity unit. Local politicians of all hues turned out. Jeremy Corbyn, a Labour MP in whose Islington North constituency the hospital is based, and the MP for Islington South and Finsbury, fellow Labour member Emily Thornberry, both oppose closure. So do the Liberal Democrats, Greens and Conservatives who will be standing in the coming general election.

The point is not to stake out different positions but to own the popular issue. The Lib Dems accuse Labour of “hypocrisy” because the plans to reorganise London health care stem from a former health minister, Lord Darzi. Labour blames National Health Service bureaucrats instead. It has become a “political turf-war”, says Shirley Franklin, co-chairman of a coalition to save the hospital.

Islington North has been a safe Labour seat for decades. The incumbent, a left-winger, last won it with a majority of over 20%. Almost all his constituents are poor, unlike the mixed picture farther south.

When the bus crosses into the constituency of Islington South, around Highbury Corner, the political and physical landscape changes. This is a constituency of extremes. It has one of the highest rates of child poverty in Britain, but houses sell for up to £3m in two of its wards, Barnsbury and Canonbury. Describing the area four years ago, Tim Butler and Loretta Lees, of King's College London, spoke of its “new group of super-wealthy professionals”. Two years later the Cripplegate Foundation, a charity, reported a “clustering” of the “exclusive” wealthy. In “Inside Islington” it said that there had been an “intensification of poverty and wealth and a widening gap between them” in large parts of Islington South.

This trend, combined with other local factors, explains the fierce battle in the constituency. Ms Thornberry has a majority of only 484 votes, and the seat is the Liberal Democrats' top Labour target in England. Their candidate, Bridget Fox, is out knocking on doors every night, with a large team of volunteers. Her message, she says, is simple: “It's pretty much, you're sick of Labour, there's no point voting Tory here and we've got good local experience on the council, so vote for us.”

But the Conservatives think they have a chance, thanks to demographic changes. Some 10% of the population leaves each year, and new housing developments are drawing in young City workers. The Tories are concentrating on wealthy Barnsbury and Canonbury but do not neglect those who live in the modest housing nearby. Their reception on the doorstep in one such ward is mixed, but many residents are keen to talk. Islington's New Tories are a novel breed: mostly young, racially mixed and self-declared “one-nation” Conservatives. Richard Bunting, deputy chairman of the local Conservative association, makes much of the fact that his candidates are “embedded”—ie, local.

Small wonder, then, that Ms Thornberry appears more than a little rattled, refusing requests to observe her on the stump. The Conservative invasion irks Ms Fox, too, who asks plaintively why the Tories can't concentrate on “their” marginals rather than chancing their arm in the Lib Dems'. Antonia Cox, the Conservative candidate, retorts that “Islington South has symbolic value—we want to park some tanks on the other side's lawn.” Islington, home to the Granita restaurant where Tony Blair and Gordon Brown once allegedly cut a deal, is well known for its links to New Labour.

The Greens are mustering too, hoping to make solid gains in local elections. They are fielding two strong candidates—James Humphreys in Islington South, a civil servant once employed in Downing Street, and his wife, Emma Dixon, a barrister, in Islington North—who may well attract disaffected voters of all persuasions.

The other powerful group that could complicate the tussle is the Islington Leaseholders Association, which is furious with both Labour and Liberal Democrat councillors whom it holds responsible for hefty repair bills on many of Islington's estates. They are putting up candidates in the council elections—and the Conservatives are wooing them for their votes in the parliamentary poll.

Most bookies back Ms Fox, but only just. Ms Thornberry's funding from Unite, a big union, could count against her if the unpopular British Airways strike it has called goes ahead. And what of Granita, one of the last stops the number 4 bus makes before it leaves the heart of Islington? It closed down a few years ago, to be replaced by a bar called Desperados.