Based on past electoral trends, the Tories need to be around three points ahead of Labour before they win the largest number of seats. In 2005, Labour won a comfortable majority of 66 seats with a lead of just three, but in 2010, the Conservatives failed to win one at all with a lead of seven. What explains this discrepancy? The most commonly cited explanation is the current constituency boundaries. Since Labour constituencies are on average smaller than Conservative ones (something that the Tories' proposed changes would have eliminated by equalising constituencies at around 76,000 voters), the former gains by requiring fewer votes to win seats. With the Lib Dems having blocked the planned reforms in 2013 (in revenge for Conservative backbenchers preventing House of Lords reform), the Tories are left to lament that the next election has been "rigged" in Labour's favour.

But while the current boundaries partially explain Labour's advantage, they do not, in fact, account for most of it. The seeming bias towards the party stems from long-standing trends that no Conservative government could automatically correct.

First, turnout in Labour seats is on average lower than in Tory ones. The working class communities that have long supported the party are less likely to vote than more affluent Tory-aligned groups. Even were constituency sizes more equal, differential turnout would continue.

Second, Labour has historically received fewer "wasted" votes than the Conservatives. Under first-past-the-post (a system supported, of course, by the Tories), votes for candidates who fail to win, or who have already won 50 per cent of the vote, are electorally worthless since they have no effect on the number of seats a party wins. Academic Danny Dorling has calculated that 1.7 million of the 10.6 million votes won by the Conservatives in 2010 were "surplus" (the highest level since 1918), increasing the party's majority in Tory seats beyond the point needed to secure MPs.

Finally, in recent elections Labour has benefited from a significant level of anti-Tory tactial voting. Lib Dem supporters in Labour-Conservative marginals (where the Lib Dems are in third place and "cannot win here") have voted Labour in order to keep the Tories out. Following the same logic, Labour supporters in Lib Dem-Conservative marginals have voted for the Liberal Democrats. The result is that the Tories find it harder than Labour to win seats with a vote share of around 35-40 per cent due to the centre-left pooling its support. By contrast, Labour is capable of winning seats with this share due to the lower level of tactical voting against it. To change that, the Conservatives need to detoxify their brand, a far harder task than redrawing the boundaries.