Page last updated at 12:39 GMT, Friday, 9 April 2010 13:39 UK

Why the Tories need a higher share of the vote to win

By Jon Kelly
BBC News

David Cameron
Turning votes into seats is a harder slog for Mr Cameron than Mr Brown

David Cameron and Gordon Brown are both in the race for No 10, but Mr Cameron's party needs to win a much higher share of the vote than his rival to win. Why?

Experts say the Conservatives need anything up to a 10 percentage-point lead on 6 May to be first across the finish line.

By contrast, Labour won the 2005 election by being just 3% ahead - with 356 seats and 35.3% of the vote, compared with 198 Conservative seats and 32.3% overall.

The Liberal Democrats - who were only rewarded with 62 seats out of the 646 in Parliament to show for 22.1% of the votes in 2005 - have long called for electoral reform.

However, the first-past-the-post electoral system which currently works against Mr Cameron's party has in fact benefitted the Conservatives in the past.

In 1951, the Tories won the most seats despite Labour winning the largest share of the vote. In 1974, the situation was reversed, with Labour winning four more seats than the Conservatives with over 200,000 fewer ballots.

This time, pundits say that a range of factors makes the electoral arithmetic more tricky for the Tories.

OUT-OF-DATE BOUNDARIES

One handicap for the Conservatives is that even newly-created seats are already "10 years out of date", says YouGov associate director Anthony Wells, who also writes the UK Polling Report blog.

The redrawing of constituencies for 2010 was based on data collected at the start of boundary review that began in the year 2000, he explains - unlike local authority wards, which are based on future projections.

Dr Roger Mortimore, Ipsos Mori's head of political research, says this works to Labour's advantage.

"Labour is strong in inner-city seats that are de-populating, whereas the Conservatives do better in suburbs and rural areas," he adds.

"As people move out of the cities, the lag in constituency boundaries benefits Labour."

LOW TURNOUT IN LABOUR STRONGHOLDS

In addition, fewer voters are needed to elect a typical Labour MP than an average Conservative representative.

"Fewer people who live in the typical Labour seat exercise their franchise," explains Professor John Curtice of Strathclyde University.

"At the last election the average turnout in the average Labour seat was 58% - in the average Conservative seat it was 65%."

DISTRIBUTION OF TORY VOTE

Conservative support has tended to be concentrated in areas where the party is strongest - meaning many of its votes go "spare".

As a result, says Anthony Wells: "In country shires, Conservatives build up useless piles of votes."

By contrast, says Prof Curtice: "The Labour Party is winning more seats with small majorities.

"But if the Tories do better in marginal seats they will improve the efficiency of their vote and the bias will be reduced."

DISTRIBUTION OF ANTI-TORY VOTE

"The anti-Conservative vote is divided efficiently," says Dr Mortimore.

"Areas where the Labour vote is strong tend to be areas where the Lib Dem vote is weak.

"There are some seats where Labour is not competitive and the party doesn't waste its time there."

Unfortunately for the Tories, such seats - largely found in the west country and the south-east of England - are where the Lib Dems tend to attract support.

Anti-Tory tactical voting was a major factor in the Conservatives' defeat in 1997.

Anthony Wells expects it to be a significant factor in 2010, too, even if Lib Dem voters have had 13 years to lose patience with Labour.

"It still happened quite a bit in the 2001 and 2005 elections," he says.

UNDER-REPRESENTATION OF ENGLAND

After the Scottish Parliament was created, the number of constituencies north of the border was cut to redress the over-representation of Scotland.

Wales, however, continues to enjoy more seats per head of the population than England - where the overwhelming majority of Conservative seats are found.

"Wales is vastly over-represented and this is safe Labour territory," says Prof Curtice.

Furthermore, he says changes to Scottish boundaries have only reduced, not eliminated, the Caledonian advantage.

Anthony Wells estimates that bringing Welsh seats into line with those in England would cost Labour roughly five seats.



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