Hung parliaments are often predicted but they seldom happen. Nineteen general elections have been held in the past 80 years: only one, in February 1974, has failed to produce an outright victor. However, this could change, as recent electoral trends have made it harder to win more than half the seats in the House of Commons.
Here is why. Half a century ago, just seven MPs wore neither a Labour nor a Conservative rosette. Either party needed to win only eight seats more than the other to govern alone. In contrast, 92 MPs were elected for the Liberal Democrats and other smaller parties in 2005. Even if the Lib Dems slip back next year, the total is unlikely to fall much below 80. If the gap in seats between Labour and the Tories is less than that, we shall wake up post-election to a hung parliament. And the gap has been less than 80 in nine of the past 16 general elections.
So, what are the prospects of a hung parliament? The chart below shows my estimation of the odds, currently roughly one in three. Let me add two health warnings. First, we don't know how public opinion will move before polling day. I have based my odds on recent polls, plus a range of plausible movements over the next six months.
Second, we can't be certain how votes will translate into seats. How will the Labour-Tory marginals behave? How will the Liberal Democrats perform?My guesses are that the Tories will achieve a slightly higher swing in the marginals than elsewhere, and that the Lib Dems will end up with around 20 per cent of the vote and hold on to some seats that the Tories hope to win back. I have built these guesses into the chart.
But whatever assumptions are made, two things are clear. First, Britain's political geography is tilted heavily in Labour's favour. Labour needs only the barest lead in the popular vote to win an overall majority: not so the Conservatives. It is quite possible for the Tories to win a million more votes than Labour, and still end up with 20 fewer MPs.
Second, the gap between the overall-majority goals is vast: anything between a Labour lead of 1 per cent and a Tory lead of 10 per cent is likely to give us a hung parliament.
Peter Kellner is president of YouGov