That will apply at the general election, and it will also be the case in next month’s local elections.

In the May 2005 general election the Tories polled just 3% less of the vote than Labour, but had more than 150 fewer MPs elected. In England they actually won more votes than Labour, but came second in seats.

Assuming a uniform movement in votes, Cameron is likely to need over 40% of the vote and be at least 10 points ahead of Labour to get a majority in the Commons. And this despite the next election being fought on new boundaries designed to eliminate any bias caused by unequal electorates in different constituencies.

Next month’s local elections see the Tories facing a similar situation in the London boroughs. In 2002, when the seats were last contested, the Tories polled a handful more votes than Labour, but emerged with over 200 fewer council seats — 654 to 866. It took nearly 900 votes to elect every Tory councillor but only 680 to elect a Labour one. And the boundaries had been reviewed immediately before the elections.

In four boroughs (Bexley, Croydon, Hammersmith and Fulham, and Merton) the Tories were even more harshly dealt with. Despite winning more votes than Labour, the party had to accept a Labour overall majority in the town hall.

Many Tory votes were “wasted” as big majorities were built up in wards where the party was safe. Labour, by contrast, won many of its own safe wards with very low numbers of votes. The average turnout in London in wards won by the Tories (35%) was above that for Labour (28%). The legacy of 2002 is that the Tories will need to be about seven points ahead of Labour in the popular vote to win more seats in the capital. This equates to a swing of 3.5% from Labour to the Tories. Anything less and Labour will be able to disguise the likely slump in its own vote by claiming it is still the leading party in London in terms of seats.

It may therefore be unfair to judge Cameron’s impact as leader by seats alone. But he does need to demonstrate the Tories are attracting new support, rather than simply picking up the crumbs from standing still as former Labour voters desert their party for the Liberal Democrats or other smaller parties.

The local arena is an ideal proving ground. Some of the Tories’ best results at the general election came in constituencies such as Ilford North and Welwyn Hatfield where there was a well known local candidate who had cultivated the electorate.

The best measure of Tory success could well be the four boroughs that got away in 2002. In Bexley and in Hammersmith and Fulham, a swing of just 2% would give the Tories control; in Croydon a swing of less than 3% is needed. Merton is a more difficult target, requiring a swing of 5%. If the Tories won three of these, and kept the eight London boroughs they currently control, they would probably emerge from the elections with control of more boroughs than Labour.

That achievement would provide a powerful counter argument to any Labour claim still to be top dog in the capital, and allow Cameron to claim he had made significant progress in overcoming the electoral “bias” against the party many see as also standing in the way of a general election victory.

The authors are directors of the elections centre, Plymouth University

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