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Who governs Britain? That is the question being put to the voters on May 6. But two other questions lurk uneasily in the background. The first is: how is Britain to be governed? The second is: will there remain a Britain to be governed, or will the election give a further push to the forces of Scottish separatism? The general election puts into the melting pot not only the first- past-the-post electoral system, with its natural accompanying single-party majority government, but also the unity of the United Kingdom itself.

Until recently, the Liberal Democrats were the only leading party proposing a change in the electoral system, and their advocacy could easily be dismissed as special pleading. Labour, however, now promises a referendum in October 2011 on the...