John Curtice: Party on a roll after best five months since Charles Kennedy quit

The Liberal Democrats kick off the party conference season this weekend with reasons to be cheerful. The party's rating in The Independent's monthly "poll of polls" has now held steady at 19 per cent for five months, its best continuous run since Charles Kennedy was deposed as leader. Unlike both its principal opponents, the party escaped the electoral fallout from the MPs' expenses scandal. Indeed in June it registered its best local election performance since 2005.

Meanwhile, Nick Clegg has begun to make a favourable impact on the electorate. His leadership ratings rose significantly following his prominent role in the Gurkhas campaign in the spring and he has not looked back since. The leadership question that has dogged the party for much of this parliament seems finally to have been put to rest.

However, there is a fly in the ointment: the popularity of David Cameron's Conservatives. At 41 per cent, the Tories' latest poll of polls rating may still be four points down on its high point this time last year, but nevertheless the party has steadily recovered much of the ground it lost during the expenses scandal. That leaves many worried Liberal Democrat MPs with a Tory challenger breathing down their neck in their own backyard.

Our map shows the 20 most marginal seats the Liberal Democrats will be defending next year. New constituency boundaries will be in force in England and Wales, so the map is based on Plymouth University estimates of what would have happened in 2005 if those boundaries had been in force then. Overall, the changes have no net effect on the number of seats the Liberal Democrats will be defending, but they do mean the Liberal Democrats will nominally be the incumbents in two newly created seats while in two others the estimates suggest the local Liberal Democrat MP will actually be starting from behind.

Of the 20 seats where the party appears most vulnerable, the Conservatives are the local challengers in no less than 13. All of these would be lost if the swing locally were to replicate the current position in the national polls. To survive, the local Liberal Democrat MP is likely to have to rely on their personal popularity and on persuading Labour supporters to vote to keep the Conservatives out. The trouble is, in many cases these sources of local support have already been exploited quite heavily.

But if the Conservatives are riding high, Labour are once again in the doldrums. At 26 per cent, the party's latest poll of polls rating is as dire as it was a year ago, when serious mutterings about the future of Gordon Brown's leadership first arose. So, perhaps whatever losses Nick Clegg suffers to the Tories next year can be compensated with gains from Labour.

Our map also shows the Liberal Democrats' 20 best second places in 2005. It shows that, contrary to popular myth, the party is no longer simply a threat to the Conservatives. No less than eight of the 20 are seats that Labour will be defending next year. All of them would fall to the Liberal Democrats if the result locally were to follow the current national swing.

It is, then, little wonder that Nick Clegg announced this week that he is setting his sights on gains from Labour. At the last election the Liberal Democrats won votes and seats in hitherto safe Labour territory in unprecedented fashion. Mr Clegg will almost certainly have to maintain and build on that progress if his party is going to be just as cheerful when it meets once again this time next year.

John Curtice is professor of politics at Strathclyde University