Parties study hard lessons of 1992 and beyond

In fact, it rocketed higher than that and the Tories still won. The lessons of that period are being studied by Cabinet ministers now. For them, the silver lining is that John Major proved it possible to win an election in a slump while borrowing like crazy.

"A lot of analysis is being carried out of how they did it," a Labour source reveals. Mr Major's decisive weapon was the threat of Labour tax rises. Mr Brown aims to turn that round and warn of Tory spending cuts. It leaves David Cameron fighting his instinct to condemn Labour overborrowing.

The 1992 election has gone down in political folklore as the one the Tories wished they had lost. Their colossal borrowing to fund welfare in the recession, and spending on pre-election giveaways, forced Mr Major to adopt such hairshirt fiscal policies - including tax rises - that the rest of their time in office was a misery.

Mr Cameron and George Osborne have spent a year preparing to take office in exactly such difficult circumstances, playing down the prospect of tax cuts and getting a grip on their juniors' spending promises.

Their best card is that Mr Brown's claim to have abolished boom and bust was plainly wrong. But if the election looks tough, the battle to recover the public finances afterwards will be much, much worse.