Professor Colin Mellors: Is it take-off or just a one-off as Clegg causes an eruption?

With a quarter of the electorate watching, Nick Clegg effectively despatched two rivals who, in his words, represented "the same old politics". The message was simple – only wholesale change can re-establish public confidence in politics.

Winning a TV debate, however, is not the same as winning an election. Do personal ratings translate into party support?

Polls that had been static for months exploded into action, with four catapulting Liberal Democrats into first place, although the latest ComRes poll retains a clear Conservative lead.

Allowing for the customary margin for error, the Liberal Democrats appear at least neck and neck with the Conservatives.

Averaging all polls since last Thursday gives Conservatives 32 per cent, Liberal Democrats 30 per cent, and Labour trailing with 28 per cent.

Shifts are most evident among under-35 year olds who appear twice as likely as older voters to have switched to Liberal Democrats.

However, the traditional vote/seat calculation, as in our chart, becomes increasingly unreliable in the context of three-party contests and wide regional variations. In Yorkshire, for example, the latest calculations suggest a seven per cent swing from Labour to the Liberal Democrats.

In our region, this would mean that their current tally of four, including their "notional seat" in the new York Outer, look safe and that they might even build on their 2008 local election successes in Hull and Sheffield.

Nationally, Conservatives have most to lose, with five of their top dozen target seats currently held by Liberal Democrats, along with 24 of the 116 seats needed for outright victory. For each of these that he fails to take, Mr Cameron will need an extra Labour seat, at a time when his direct lead over Labour is diminishing. If, as seems likely, the Liberal Democrats improve on their 2005 vote and pick up additional seats, then the Conservative task becomes still harder.

However, it is not all good news for Liberal Democrats. Even with 30 per cent of the vote, Nick Clegg would probably only win 100 seats.

It is plausible that the Conservatives could emerge with the largest share of the vote, Liberal Democrats in close second place, and yet leave third-placed Labour with the largest numbers of seats – a recipe for political and constitutional uproar.

With the campaign mid-point reached, polls still suggest that it is leadership appeal, rather than policy content, that is winning over undecided voters. The challenge for Nick Clegg in the second TV debate is to show that last week was take-off, and not a one-off, for his party.

Professor Colin Mellors is a political scientist at the University of York.