Labour and Conservatives need to make frantic calculations to become the largest party in May

With less than six weeks to go until polling day, Labour and the Conservatives remain level-pegging in the polls, depending on which you read, and the precedents from previous parliaments do not look promising for either party in terms of achieving the breakthrough they need.

Working out who will be the largest party after the election is a lesson in mathematics, says Rick Nye
Working out who will be the largest party after the election is a lesson in mathematics, says Rick Nye
By now the last three principal opposition parties that went on to govern – the Conservatives in 1979, Labour in 1997 and the Conservative again in 2010 – were comfortably outperforming where Labour is now, which is at about the same level as Michael Howard's Conservative Party was before the 2005 election. 

Nor is the news better for the Conservatives.

The last two times an incumbent government was re-elected – Labour in 2001 and 2005 – it had far more than the one third of the electorate who currently say they will vote Tory backing them.

Rather than asking who is going to win on 7 May, it probably makes more sense to focus on who is likely to emerge as the largest party. 

Here the relationship between poll leads and who might be Prime Minister after the election is not as straightforward as it might seem.

Despite the losses, Labour is likely to suffer at the hands of the SNP in Scotland. Its advantage over the Conservatives in turning votes into seats and its potentially greater choice of coalition or confidence and supply partners means that Ed Miliband can afford to fall behind the Tories in the polls, probably by up to three per cent, and still end up as Prime Minister.

In the final days of the election campaign, it will be tempting to focus on 'trophy seats' in search of a repeat of that 1997 Portillo moment. 

Can Nick Clegg hang on in Sheffield Hallam; will Nigel Farage win Thanet South; what will be the fate of the two Alexanders – Douglas and Danny – at the hands of rampant Scottish nationalism? 

These contests may enliven the small hours of Thursday night and Friday morning but the shape of what emerges in the days and weeks that follow will be determined by two distinct battlegrounds.

How much of the ground lost to the SNP that Labour can recover in Scotland; and how many seats the Conservatives can successfully defend in England and Wales that currently appear lost to Labour. 

As a rough guide, if Labour can gain 40 seats or more from the Tories south of the border it should emerge as the largest party; if it manages fewer than 35 it probably won't unless it recovers strongly in Scotland. 

Even then the Conservatives could end up with the most votes and largest number of seats and still not form the next Government because Labour is likely to have more choices of coalition or confidence and supply partners. 

At this rate things could get messy and stay messy for quite some time. 

Rick Nye is a director at Populus

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