John Curtice: All still to play for as a Tory majority is far from assured

Only one thing seems reasonably certain about what will be unveiled when the ballot boxes are opened on Thursday night: the Conservatives will win most votes. Beyond that, all is uncertainty and potential flux.

The latest clutch of polls, all conducted since the last of the three leaders' debates, suggests the Conservatives have edged a little further ahead. Their average poll rating is now 35 per cent, up one point on their position across all polls conducted between the second and the last leaders' debates. Liberal Democrat support has fallen back by a point for the second week in a row. Although most of the fallout from Nick Clegg's success in the first leaders' debate is still in evidence – at 28 per cent, his party's support is still as much as seven points up on before that debate – it has all the appearance of a slowly ebbing tide.

Labour's support, meanwhile, seems firmly stuck on just 27 per cent of the vote, leaving the party at risk of emerging with an even lower share than Michael Foot secured in 1983. Still, given the events of last week this might come as something of a relief to Labour. It appears that, despite the intense media frenzy the Prime Minister's encounter with Gillian Duffy in Rochdale generated, it has had no discernible impact on his party's fortunes (though the same may not be true of Mr Brown's personal popularity). Meanwhile, thanks to the decline in Liberal Democrat support, Labour now appears to have as much as an even chance of avoiding third place in votes.

Much rests on this apparently highly uncertain contest for second place in votes. If the Liberal Democrats can stay ahead, the argument for electoral reform, as well as the party's long-term prospects, will receive a substantial boost. But if Liberal Democrat support ebbs further away, then Thursday night could prove a damp squib for Mr Clegg – much as the Alliance's close third place in 1983 proved to be.

The slight improvement in the Conservatives' position has been in evidence for some days – polls conducted wholly between Monday and Thursday's debate were already putting the party on 35 per cent, on average. Nevertheless, it has helped create a sense of momentum for the party. And if Mr Cameron is to win an overall majority, he badly needs to advance further during the final four days of the campaign.

Despite the Tories' now substantial lead, the latest polls still point, on the conventional calculations, to a hung parliament in which the Conservatives are some 44 short of a majority – and probably unable to form a government without some understanding with Mr Clegg.

However, if the Tories could add another two points to their national tally, and at the same time secure an additional one-point swing in marginal Labour-held seats – as the latest polls of such seats suggest could well happen – they might just be home and dry.

They might be aided by the fact that voters are apparently coming to think of Mr Cameron as a likely Prime Minister. YouGov reports that 40 per cent felt he was the most "prime-ministerial" in the final leaders' debate, up 12 points and ahead of Mr Brown for the first time. But, equally, if in the last few days there were just a one-point swing back from the Conservatives to Labour, Mr Cameron could still find himself nervously waiting until late on Friday to see whether he has most seats. The result now rests on a knife-edge.

John Curtice is professor of politics at the University of Strathclyde. He is analysing election opinion polls for 'The Independent'