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Rise of Lib Dems is worry for Cameron

John Curtice, the renowned psephologist who will be analysing the polls throughout the campaign for <i>The Independent</i>, tests the mood as Brown prepares to call the election

Gordon Brown will go to the Palace tomorrow in the knowledge that his party faces an uphill battle. The Tories' average poll lead, which a few weeks ago had narrowed to as little as five or six points, has now widened once more to eight points, following the Budget and the Conservatives' promise to scale back next year's increase in national insurance.

Labour's recovery in the polls was already beginning to look rather fragile even before Alistair Darling got up in the Commons chamber with his Budget box. From a brief high point of 33 per cent at the end of February, the party had slipped back to an average rating of just 31 per cent.

Yet despite praising Mr Darling's performance at the dispatch box, voters gave the Budget the thumbs-down, most polls found. And once they had had a day or so to assess its contents, Labour's support edged down yet further to just 30 per cent – at which low point it continues to rest.

George Osborne's announcement on national insurance, welcomed by business leaders, has been followed by some improvement in the Conservative rating – up one point to 38 per cent. However, the party is still down on its position at the beginning of the year when its average poll rating was 40 per cent. The announcement has not yet been a game changer.

The Conservatives are not the only party to have made progress. The Liberal Democrats have edged up a point too since Vince Cable was widely thought to have emerged ahead in Channel 4's Chancellors' debate. The party is now at the 20 per cent mark for the first time since its party conference last September.

Nick Clegg's troops now look as though they will enter the election campaign in almost as strong a position as they started the 2005 contest. According to a YouGov poll, over half (54 per cent) think he is doing well as leader for the first time ever.

This apparent Liberal Democrat advance will worry David Cameron. An eight-point lead over Labour might just be enough to secure the Tories a narrow overall majority if, as several polls suggest, they secure an above-average swing in marginal seats held by Labour. But it will not be sufficient if at the same time the Liberal Democrats manage to fend off the Conservative attack on their key marginal seats.

In any event, if the swing since 2005 were to be uniform across the country, the latest poll figures point to a Tory tally of 303 seats, Labour 263, Liberal Democrats 51 and others 33.

John Curtice, professor of politics at Strathclyde University, will produce a poll of polls for The Independent on Mondays.