Andrew Grice: Now the Tories have gained the upper hand

After John Major defeated Neil Kinnock at the 1992 election, The Sun claimed it had "won it". Yesterday, its sister paper The News of the World declared it had stopped the 2007 election that never was.

In fact, Gordon Brown took the decision to call it off before he knew the results of the paper's ICM poll in 83 marginal seats showing a six-point Tory lead. Labour's polling in the marginals was less gloomy but already causing concern in Downing Street since the Tories unveiled a plan a week ago to raise the threshold for inheritance tax to £1m.

The message from the marginals was that people liked the Tories' tax cut and believed the pledge was "so big" the Tories would have to deliver on it. His aides concluded that Mr Brown would need more time to demolish the Tories' plan.

On Friday, Mr Brown's pollsters, Deborah Mattison and Stan Greenberg, told him he would win an election now but couldn't be sure of the margin, that his working majority of 69 could be cut to 30 in a worse-case scenario – hardly the decisive mandate he wanted.

On Saturday morning, the Prime Minister knew the time had come to bite the bullet. "Let's draw a line under it. Let's get out and do it," he told aides.

The endgame was messy. After the BBC's Andrew Marr was summoned to Downing Street for an interview, Tory spin doctors tipped off other broadcasters, who camped outside No 10, miffed that the Prime Minister was not going to speak to them too.

His announcement was even forecast on the Tory grassroots ConservativeHome website at 3.22pm, an hour before the Marr interview. David Cameron was able to do long interviews on the live TV channels attacking Mr Brown, while only short clips of Mr Brown's interview for Marr's Sunday morning BBC programme were released on Saturday. Labour was outspun on a story that should have been under its control.

Perhaps Mr Brown's biggest mistake was to assume that allowing election fever to spread would add to the pressure on Mr Cameron. It had the opposite effect, forcing reluctant Tories to rally behind their leader.

Experienced Labour hands are wondering how the Prime Minister allowed himself to the brink of calling an election. At a private party on Saturday, attended by several Labour bigwigs from the capital, the former cabinet minister Frank Dobson caught the mood when he spoke of "relief" the poll was off.

Close allies insist Mr Brown was "never persuaded" and had probably made up his mind not to call one by the close of Labour's conference in Bournemouth – despite opinion polls giving his party a lead of up to 11 points. He declined to rule out an immediate poll because he wanted to keep the heat on the Tories, believing this would force them to rush out policies at their conference in a desperate attempt to head off an election they did not want.

Although the Tories insist they were always going to make the inheritance tax announcement, the only crumb of comfort for Labour is to have "flushed out" some of the opposition's key policies at least 18 months before the election takes place.

There is no doubt that the non-election has weakened the Prime Minister. He made "strength" part of Labour's conference slogan after leading the nation through attempted terrorist attacks, foot and mouth, and floods in a solid first 100 days. Yesterday, jubilant Tory officials handed Westminster journalists bottles of Brown ale bearing the words "Bottler Brown".

What matters is how it plays outside the Westminster village. Labour hopes it will be a five-minute wonder, that the voters will barely remember something that, after all, did not happen. I suspect the Tory tax cut plans – and, crucially, whether they stack up – will have a greater bearing on the next election.

It is too early to judge whether Mr Brown's dismal weekend is a pivotal moment, the turning of the tide for which the Tories have been praying since 1997. But momentum is a crucial ingredient in politics. Mr Cameron now has it, and Mr Brown needs to regain it quickly. The rules of the game have changed.

a.grice@independent.co.uk