The distinctly tarnished victory that Labour won last Thursday was just one reflection of what may be a new era of electoral politics in Britain. With turnout refusing to budge much above 60%, despite the controversial liberalisation of the postal voting rules, Tony Blair won the positive support of little more than one in five of all those eligible to vote. Never has an elected British government’s mandate been so thin.

Out, too, went the traditional swing of the pendulum from an unpopular government to the official opposition. Not only was Labour’s share of the vote (36%) the lowest received by a party winning a majority in the House of Commons, but the Tories’ 33% share also marked the third election running at which they plumbed depths not experienced since the 1850s. Together the two main parties polled less than 70% of the total vote for the first time since the genuine three-party election of 1923.

The election also drove a further nail in the coffin of uniform swing. Labour’s majority would have been nearly 100 if the movement in all constituencies had been the same.

Instead, the party was pegged back by both regional variation (it did especially badly in London) and some dramatic individual results. Apart from Bethnal Green and Bow and Blaenau Gwent, double-figure swings ousted Labour MPs in places such as Cambridge, Hornsey and Wood Green and Manchester Withington.

Yet again, however, Labour benefited from the operation of the electoral system to win a far disproportionate number of seats for the votes it received. On Thursday, Labour elected one MP for every 27,000 votes. By contrast, the Tories got one MP for every 44,000 votes and the Liberal Democrats one for every 100,000 votes.

New boundaries will come into force in England and Wales before the next general election, but they will only mitigate the bias towards Labour. By 2009-10 these new seats will already be almost 10 years out of date and, with turnout in safe Labour seats continuing to be low, the party is almost bound to win more seats for any given share of the vote than its rivals.

On current boundaries it would take a swing of just 1.9% from Labour to the Tories for Labour to lose its overall majority. The pivotal seat is Dorset South which, despite being the most marginal Labour seat in the last parliament, the Tories failed to win last week.

However, for the Tories to become the largest party the task is rather more formidable. They would need to win Wirral South (first captured by Labour in a by-election before the 1997 general election) on a 4.7% swing.

A fragile overall majority for the Tories of two would take a swing of 7.5% — more than Margaret Thatcher achieved when winning in 1979. A comfortable majority of 50 — itself less than what Blair won on Thursday — would require an improbable 9.1% swing and the capture of Middlesbrough South and Cleveland East.

So it is still the case that the Tories must poll at least 40% of the vote to get back into power.

The Lib Dems made some progress at Labour’s expense and have more MPs than at any time since Lloyd George. However, they may now find themselves having to woo electors with rather contradictory agendas. Their share of the vote actually declined in seats where they were close challengers to the Tories — hence the failure of the “decapitation” strategy aimed at senior Conservative politicians — and they fell back, too, in places that they had won from the Tories in 1997. The southern English middle classes were not enamoured by talk of higher taxation and a local income tax.

On the other hand, the Lib Dems forged ahead in other very different types of seat. The swing from Labour to the Lib Dems in the 50 constituencies with the highest Muslim population was 8.5%.

Even Clare Short, who resigned from the cabinet over the Iraq war, was not immune, suffering a swing of 20% to the Lib Dems in Birmingham Ladywood. In similar constituencies in London, the Labour vote was down by more than 10% with Respect picking up significant support in both East and West Ham and Poplar and Canning Town.

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