The Liberal Democrat victory in Brent East is not the biggest defeat that the party or its predecessors have inflicted on Labour. But it is undoubtedly the most significant.
The 29 per cent swing from Labour to the Lib Dems is well short of the 44 per cent swing when Simon Hughes won Bermondsey in 1983. It does not even match the 32 per cent swings in Birmingham Ladywood in 1969 and Liverpool Edgehill in 1979. But the Ladywood and Edgehill victories were the result of pavement politics, while in Bermondsey the party profited from the controversy that surrounded Labour's candidate, Peter Tatchell, the gay rights campaigner.
There were no particularly pressing local issues to influence voters in Brent. Labour's candidate, Robert Evans, is not someone whom half the party would want to disown. After all he is already an MEP. Yet Labour suffered only its sixth postwar by-election defeat at the hands of Britain's main third party. So the defeat is not simply a little local difficulty. The finger of blame lies with the party nationally. And there are two obvious candidates - Iraq and the domestic agenda.
How much trouble Iraq is causing Tony Blair across the country as a whole should not be exaggerated. More people still think we were right to go to war than not. And at -33 Mr Blair's current Mori poll satisfaction rating is only three points worse than in February before Britain went to war.
However, in a constituency such as Brent East, with a substantial Muslim and ethnic minority population, the war is likely to have been especially unpopular. And there are plenty of Labour MPs whose electoral fortunes depend in part on the votes of Britain's ethnic minority communities.
Iraq is part of a wider story. In going to war on a prospectus many have come to doubt - twice as many think the Government embellished the case for war as think it did not - Mr Blair has fuelled doubts about the ability of the Government to deliver on its promises that existedwell before British troops invaded Iraq.
In March this year Mori found that only 36 per cent believed Labour would improve public services. This was well down on the 54 per cent who thought they would at the time of the 2001 general election. What has happened post-Iraq is simply that this decline has continued. And if blame for the by-election defeat does lie with Mr Blair and his Government then the Brent East by-election heralds a new era.
Hitherto under Mr Blair's leadership Labour has enjoyed double-digit poll leads and has only lost elections such as the 1999 Euro elections that few MPs and even fewer voters cared about. Whatever they thought of his policies Mr Blair could always tell his party they were the key to electoral success.
But the double-digit poll leads disappeared some time ago. And now Labour has lost an election that MPs care about, that cannot be blamed on local difficulties and where at 36 per cent the turnout was higher than it has been in any by-election held in a safe Labour seat since 1997. Labour MPs may now begin to doubt Mr Blair's electoral touch. And if they do they are likely to question his policies too.
John Curtice is professor of politics at Strathclyde UniversityReuse content