Labour appears to be on the verge of an unprecedented third consecutive general election victory. Despite Iraq and the collapse in Tony Blair’s personal popularity ratings there is no sign of the straight two-party swing that tends to be necessary for a decisive change of government in Britain.

When Blair won in 1997, and Margaret Thatcher did so in 1979, there was a clear shift in public mood and electoral support from the government of the day to the major opposition party. This time the Tories are struggling to push their support beyond the mid-30% level where they have drifted since Black Wednesday in September 1992. The Tories also continue to lag behind Labour as the favoured party on key election issues such as health, education and the economy.

All the indicators from this parliament confirm the Tories’ predicament. In six parliamentary by-elections, all in safe Labour seats, Labour’s support has dropped by an average of nearly 20%.

But in no case have the Tories been the beneficiaries — their vote has declined by an average of 4% and in Hartlepool they were beaten into fourth place by UKIP.

In the European parliament elections in 2004 the Tories made much of their electoral success, topping the poll in the first nationwide test of Michael Howard’s leadership. But their share of the vote was just 27% — some 10% less than at the equivalent contests in 1999.

The Tories have also been out in front in local elections since 2003 but their share of the vote has been no better than that under William Hague in 1997-2001.

Instead of shifting to the Tories, electors have been registering their dissatisfaction with the government by voting for the Liberal Democrats or for one of a range of minor parties. The Lib Dems’ average poll rating in the first three months of this year has been more than 20% (during the same period in 2001, when the pollsters’ methodology was slightly less favourable to them, it was just 14%), and the party has been neck-and-neck with Labour in the contest for second place in local elections.

The combination of the Greens, UKIP, the BNP and Respect took almost three in every 10 votes at last year’s European contests. Indeed they were the first modern elections at which Tories and Labour combined polled less than half of all votes.

This dispersal of the anti-government vote is likely to help save Blair. The Tories are the main challengers to Labour in 135 of its 150 most marginal seats. Any movement from Labour to any other party makes it harder for the Tories to win. A direct 3% swing from Labour to the Tories in Harwich and the seat would change hands. A 3% swing from Labour to the Lib Dems, whether prompted by Iraq or the unwinding of previous tactical voting, and Labour would hang on.

It is possible that not everything is as rosy as it appears for Labour. The party’s poll lead is in single figures; a month before the elections in 1997 and 2001 it was close to 20%. On both those occasions Labour did less well at the election itself.

If history repeats itself then the result could be quite close in terms of votes polled. Labour would lose two or three dozen marginals and the Tories could register some surprises in places such as Rossendale, Darwen and Swindon where they have done well in local elections.

But Labour, based on the efficient distribution of its vote in the seats that matter, is likely to win many more seats for any given share of the vote than the Tories. Assuming a uniform swing across the country, a tie in votes would see Blair returned with an overall majority of nearly 60; even a Tory lead of 2% in the popular vote would still leave Labour well out in front in the Commons.

Only if the Tories are 10 points ahead of Labour can Howard be guaranteed an overall majority, and it would be smaller than the 21 majority that caused John Major so much trouble in 1992-1997.

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