John Curtice: There is a silver lining for Labour: the Tories are suffering as well

Today's elections were never going to be easy for Labour. It started the campaign with an average opinion poll rating of only 27 per cent. At that level of popularity, all four of the county councils it is defending looked highly vulnerable. And with smaller parties always likely to attract votes on a scale unimaginable in a Westminster election, it seemed the best it could hope for in the European elections was to repeat the disastrous performance of 23 per cent it recorded last time.

But after the expenses scandal the party's poll rating has almost been in freefall. On average just 21 per cent now say they would vote Labour in a general election. Even if some of that is a temporary loss occasioned by the unusual prominence of smaller parties during a European election, it is certainly in at least as much electoral difficulty now as it was in the wake of the 10p tax row this time last year.

So not only do the local elections now threaten Labour with an unprecedented whitewash, but the outcome of the European elections could destabilise the party too. The last four polls of Euro voting intentions put Labour on an average of just 18 per cent, with both the Liberal Democrats (16 per cent) and Ukip (15 per cent) apparently breathing down its neck for second place.

Falling below 20 per cent could just be the tipping point that persuades Labour MPs that Mr Brown needs to go. Certainly Mr Brown's personal poll ratings give MPs every reason to believe that public perceptions of their leader are contributing to their electoral difficulties.

If there is a silver lining for Labour it is that the past four weeks have been torrid for Mr Cameron and the Conservatives too. The Opposition leader has been rowing rapidly to ensure his party's electoral boat is not swept away by its involvement in the expenses row. Meanwhile, he has discovered that his Eurosceptic tone has been insufficient to stem a considerable loss of support in the European elections to Ukip.

There now seems to be little prospect that the Conservatives will register in the European elections the kind of performance that would demonstrate the party is set for victory when the next general election does eventually happen. At 28 per cent, the Tories' average European poll rating is only a little higher than the 27 per cent it won five years ago. It is well short of the 36 per cent William Hague managed in 1999. But perhaps an even bigger worry for Mr Cameron is that his party's performance could now fall short of expectations in the local elections. Over the past month his party's support has fallen by at least three points in polls of Westminster voting intentions – and by even more according to the most recent ComRes poll for this paper. With Ukip contesting nearly one in four of the 2,300 local seats at stake, the Conservatives will fear some of its support could seep away to Nigel Farage's party in the local elections too.

As a result, the Conservative performance could fall well short of the 44 per cent it secured last year, when the local votes are projected into a nationwide vote. Mr Cameron will not want the impression formed at this stage of the electoral cycle that his party's support has begun to recede.

While both Labour and the Conservatives have suffered from the events of recent weeks, the Liberal Democrats have largely emerged unscathed. Their Westminster support has held steady at around 18 per cent, while Mr Clegg's personal ratings have improved dramatically, thanks not least to his involvement in the Gurkhas campaign. However, this still means the party is less popular than it was on general election day in 2005, which is when the county council seats at stake today were last contested.

The Liberal Democrats' relatively Europhile stance and vulnerability to any surge of support for the Greens means they have often struggled in European elections. The party will feel queasy that, with the Greens running at 11 per cent in the polls, up five points on 2004, it could yet suffer the same fate again. Perhaps all the parties at Westminster will all end up hoping we forget today's elections as soon as possible.

John Curtice is professor of politics at Strathclyde University