If the last election had been fought under AV or in just 600 constituencies...

By Tim Montgomerie

If all goes the Tory way the number of MPs in the Commons will be reduced to 600 by the time of the next election.

If Matthew Elliott - confirmed overnight as head of the anti-AV referendum campaign - is unsuccessful the first-past-the-post electoral system will be abolished and replaced with Alternative Vote.

Two analyses this weekend attempt to calculate what would happen if the last election had been fought under AV and alternatively if it had been fought with the number of constituencies reduced to just 600. The table below summarises what might have happened:

The Tories would have won disproportionately fewer seats under AV and as Mike Smithson blogs on PoliticalBetting a Lib/Lab pact would have had a majority. His guess "is that the yellows would have gone, after some hard bargaining, with Labour." Cameron will hope that the Lib/Con Coalition will alter preferences by the time of the next election and make AV less negative for the Conservatives.

An article in The Sunday Times (£) by Colin Rallings and Michael Thrasher looks at the effect of reducing the number of MPs to 600. They conclude that the change helps the Tories but not by nearly enough to give them a majority. The unequal seat size is only "10%" of the explanation, they write, for the Labour advantage of 63 seats under the current system:

"There is a common misconception that the independent periodic boundary reviews should remove electoral bias. This view is mistaken, because such reviews are only concerned with one of the elements that contribute towards bias: unequal electorate size, or malapportionment, as it is sometimes described. But other elements within the voting system are contributing more towards overall bias. These are: vote distribution (geography), differential turnout (abstention), and the effects produced by competition from smaller parties. The boundary review will solve the problem of malapportionment, but it will largely ignore the bias produced from the remaining elements."