John Curtice: The roles are reversed after this result - now it's Cameron’s turn in the hot seat

Until Thursday night, both Nick Clegg and Ed Miliband faced unease within their parties.

David Cameron, by contrast, was still riding reasonably high. After Oldham those roles will, for the time being at least, be reversed.

Mr Miliband has struggled to make a favourable impression on the public – of recent opposition leaders, only Michael Foot and William Hague had worse personal poll ratings after three months in the job. So the Labour leader needed a good win in Oldham to allay fears that his apparently lacklustre performance was harming his party at the polls.

Oldham duly delivered. Currently the national opinion polls put Labour some 10 points or so above where they were last May. And in Oldham, in real ballot boxes, Labour's share of the vote increased by the same margin. Indeed, with 42 per cent of the vote, the party was back locally to where it had been when it last won a general election, in 2005.

Mr Clegg, meanwhile, has seen his party take a hammering in the polls since it entered into coalition with the Conservatives. Many a recent poll has put the party's standing below 10 per cent. The decisions to sign up to the Tory position on cutting the deficit and to reverse the party's position on tuition fees were sharply at odds with what many of its supporters expected.

But in Oldham there was no sign of the meltdown suggested by the national opinion polls. The party fully retained the 32 per cent share of the vote it won last May. True, in similar circumstances in the past – including in next door Rochdale in 1972 – the party has pulled off spectacular by-election successes. But at least those Liberal Democrat councillors and politicians in the Scottish Parliament and Welsh Assembly who have to face the electorate in May have been given an apparent ray of hope – and Mr Clegg some much needed breathing space.

Still, the Liberal Democrats will want to ponder the apparent reason for their success – a spectacular collapse in the third placed Conservative vote. Tory spokespeople were keen to argue yesterday that parties that start off in third place typically get squeezed in by-elections.

However, the 14-point drop the Tories suffered on Thursday was larger than anything they have suffered in previous by-elections where they started off third behind Labour and the Liberal Democrats – including the 10-point drop they suffered in 1972 in Rochdale.

Evidently, as opinion polls published last weekend suggested, Conservative voters in Oldham switched tactically to the Liberal Democrats in unprecedented numbers – clear evidence of how the formation of the Coalition has started to rewrite some of the rules of the British electoral game.

Together with UKIP's relatively rare success in saving its deposit, the Tory reverse will provide ammunition for those on the party's right who fear the consequences of the Coalition.

Tactical voting in Oldham also means that Mr Clegg's respite could easily prove short-lived. Liberal Democrats facing Conservative opponents in May cannot expect the favour to be repeated. Those fighting Labour may find that tactical support from erstwhile Conservatives is not enough to save their skins. Mr Clegg needs to use his breathing space well.

The writer is Professor of Politics at Strathclyde University