David Cameron must secure a national swing of 6.9 percent from Labour to the Conservatives to win the election with an outright majority, a feat no party has achieved since 1945.
In the general election of 2005 the Conservatives won a quarter of a million more votes than Labour, but had 92 fewer MPs in England.
Radio 4's More or Less presenter Tim Harford and Lord Patten, former chairman of the Conservative Party, examine whether the electoral system is biased against the Tories.
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