In 2010, with 36% of the vote the Tories got 305 seats (47% of total), Labour with 29% of votes got 258 seats (40%), the Libs voteshare of 23% delivered just 57 seats (9%).
From this, we can see why the two big parties are so keen on the present system and also why the Tories are unhappy with how their spoils from the biased system are divvied up with Labour.
Opinion polls currently show Labour on around 40%, and Tories around 33% of the vote. I think it inevitable that Labour will do better than Gordon Brown's dismal 29%, but the Tories will struggle to get more than 36%.
So lets assume that at the next general election both get around 35%, with the Libs around 15%.
Under the current boundaries this would deliver around 300 seats to Labour, 290 to the Tories and around 30 to the Libs, with 30 others.
The others are split fairly evenly left and right so cancel out. Lib/Lab would have 330, a fragile majority of just 15 (as Sinn Fein don't take up their seats).
But under the new Tory proposed boundaries, the same votes would deliver 305 Tories, 250 Labour, 20 Libs and 25 others. This would give the Tories a 15 seat majority to govern alone. No wonder the Tories are so desperate for the changes.
In both situations Tory losses to Labour would be alleviated by Lib losses to Tories. The new boundaries hit Labour and Libs because of the loss of Scottish and Welsh seats. And Labour further lose out in England because of the way previously urban seats have been made more rural by the increase in size.
It will be near impossible for Cameron to get this through without Lib support. In the Commons the maths is tight but against him. He needs iron discipline from his 305 MPs even when some stand to lose their seats. He will need sufficient bribes for them. Plus he needs most of the Scots and Welsh Nats and the NI unionists. Even this would only bring a majority of one. Of course some Labour and Libs might vote for the changes though the opportunityvto defeat the government would probably be a stronger one.
The SNP would support him for concessions on the independence referendum and Scots parliament powers. Plaid Cymru are dead set against the changes as Wales loses a quarter of its seats, so Cameron would have to offer something on these changes as well as more powers to the Welsh assembly. Both high prices to pay.
And even if the Commons vote was won, it would be stopped in the Lords with probably insufficient time to guillotine before an election.
So, even Cameron must know these changes will be defeated. The Tories will use the vote as a propaganda exercise to win public support and maybe even as a platform for coalition break-up and an early election. Which probably explains why the Scots independence referendum in 2014 is deemed so inconvenient. Interesting times indeed.