Professor Colin Mellors: In this tight poll battle, there may be more surprises ahead

Six months ago, it looked very different. Apart from a brief "bounce" in late summer 2007, for almost all of Gordon Brown's premiership Labour has lagged behind the Conservatives by at least 10 points – the level needed for outright Conservative victory. The gap started to narrow in January and, during the campaign itself, only seven of over 80 polls published have given Conservatives a double-figure lead over Labour, mostly in the first week of campaigning.

Before the first leaders' debate, Conservatives were averaging 39 per cent in the polls, Labour 31 per cent, with Lib Dems struggling to reach 20 per cent. Subsequently, the Conservative share has dropped to the mid-thirties, with Labour and Lib Dems fighting for second place in the high-twenties for most of the last fortnight.

Apart from a handful of polls in the aftermath of the TV debate, Conservatives have led throughout and remain on course to win the largest number of votes, albeit perhaps with a share only slightly higher than in 2005.

The real difficulty is translating votes into seats. There is also the potential impact of regional variation, with the latest PoliticsHome analysis of YouGov figures suggesting that, in Yorkshire, Labour is faring much worse than is suggested in national polls with huge adverse swings of 8.5 per cent and 11.5 per cent to the Conservatives and Lib Dems respectively.

This means that up to 11 current Labour-held constituencies could be vulnerable to a Conservative challenge, and would put Lib Dems in sight of three more seats, at Labour's expense.

Significantly, the field work took place between April 26 and last Sunday. A note of caution is required – the most recent polls show a modest firming of Labour support and a small decline in that of the Liberal Democrats.

Even this late, a Conservative majority is still entirely possible. However, if, tomorrow morning, there is electoral balance, much will depend on the precise figures.

A situation where the Conservative Party was above, or around, 300 MPs, with Labour in third place in terms of votes would be very different to one where Conservatives were well short of 300 seats with Labour second in votes and seats.

In the former situation, it would be entirely plausible for David

Cameron to go it alone in a minority administration.

By contrast, what Nick Clegg decided in the latter situation might determine not just who gets the keys to Downing Street but a whole series of subsequent political and constitutional changes. This is one contest where it is almost as important who comes second as who comes first.

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