General Election 2015: Tories pick up in the polls - but they still need to make progress

The Conservatives' one-point lead will not necessarily be enough to make them the largest party

David Cameron addressing staff at the Asda headquarters in Leeds (PA)

This has been a relatively good week for the Conservatives in the polls. But they will have to make more progress yet – or hope that some of the pollsters at least are wrong – if David Cameron is to retain the keys to Downing Street.

Our latest poll of polls, based on all of the polls published in the last week, puts the Conservatives ahead for the first time in the campaign, albeit only by one point. The party’s support has edged up to 34%, while Labour remain on 33%.

However, on its own a one-point lead will not necessarily be enough to make the Conservatives the largest party. Thanks to the failure of the party to get the constituency boundaries redrawn, Labour voting seats contain fewer registered voters than Conservative voting ones, while the turnout in Labour seats tends to be lower too.

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As a result, and despite Labour’s continuing difficulties in Scotland, the party could still win 290 seats to the Conservatives 275. The balance of power would be held by 50 nationalists and Greens, who are committed to voting Mr Cameron out of power.

However, this assumes the swing to Labour in the key Tory-held marginals matches the 4% swing the polls currently anticipate across England and Wales as a whole. But this may not prove to be the case.

A poll of Tory held marginals conducted by ComRes this week suggests the swing to Labour could be between a half and one point lower in Tory held marginals. Lord Ashcroft’s polling points to a similar conclusion, at least in those marginals being defended by an incumbent Tory MP for the first time.

If the swing to Labour is a point lower in Tory held marginals, Labour’s projected seat tally, at 277 seats, slips below that of the Conservatives on 279 - though the nationalist block would still be able to keep the Conservatives out of power.

But there is an important discrepancy between the polls. Those conducted by phone, which have very much been in the minority, have on average since Easter put the Conservatives three points ahead. In contrast the two parties have been tied on 34% each in internet polls

Unlike a one-point lead, a three-point one would be sufficient to put Mr Cameron clearly in pole position. Much rests on which polls are right – and on the last week.

John Curtice is Professor of Politics, Strathclyde University