Politics: The Liberal Democrats hold the key to boundary reform – and therefore the next election

The Cameroons believe they have come up with a policy that will deliver at least 20 more Tory seats at the next election. No. 10 is, understandably, determined to get this measure through: nothing else promises them anything like the same electoral dividend. But this silver bullet isn’t a tax cut or a new approach to Europe. It is a set of changes to the constituency boundaries.

This boundary review has been designed to limit the pro-Labour bias in the Westminster electoral system. It reduces the number of parliamentary seats from 650 to 600, which notionally reduces the number of Labour MPs by 28.

If its recommendations are enacted, the effect on British politics would be dramatic. The pollster Anthony Wells calculates that if an election were held today on the current boundaries, Labour would be two seats short of an overall majority. When you consider that there are five Sinn Fein members who don’t take their seats, that translates into a majority of sitting MPs. If, however, the election were held on the proposed new boundaries, the Tories would be two short of an overall majority. This would allow them to govern alone if they wanted.

Even under the new system, Labour would still have an advantage. The Tories would need, assuming a uniform national swing, to be seven points ahead of Labour to win an overall majority. At the last election, however, they needed a double-digit lead to achieve a majority.

But the Prime Minister now faces a growing risk that the reform will fall victim to the tensions within the coalition. The last election convinced Cameron and his circle that, under the current system, it is almost impossible for the Conservatives to win outright.