Tactical voting - whereby voters cast their ballots not on the basis of the party they want, but in order to prevent another party from winning - can play havoc with projections. In 1997, tactical votes are estimated to have cost the Conservatives around 40 seats; they also played a part in the 2001 and 2005 election results.
At this early stage of election preparations, it is difficult to measure what the impact will be. Generally, tactical decisions are not made until a campaign is nearing its end, when the specific issues in any given constituency have come to prominence. In addition, there may be confusion this time around over boundary changes in England and Wales.
The scale of tactical voting can be significant. On the eve of the 2005 election, ICM found that 10 per cent of those voting for Labour, 16 per cent of Conservative voters and 30 per cent of those supporting the Liberal Democrats had made their decision on that basis (see graph, below right). But in 2005, the outcome was almost a foregone conclusion. Now, things are very different, especially for Labour and the Tories. In key battlegrounds there will be major battles to win the votes of other parties' supporters, mostly the Liberal Democrats.
How might tactical Lib Dem supporters vote? Responses to YouGov's regular "forced choice question" - which gives voters only two options, a Cameron-led Conservative government or a Brown-led Labour one - give an indication. In the most recent YouGov/Sunday Times poll, the split was 43 per cent Tory, 38 per cent Labour. Among Liberal Democrat voters, however, 43 per cent said they would back Brown's Labour, while just 26 per cent opted for Cameron's Tories (see chart, above). This may be an advantage Labour can exploit.
In previous elections, Labour supporters have generally been more ready to switch to the party best placed in their constituency to beat the Conservatives than Tory voters have been to keep Labour out. In the past, this has been a critical factor in helping Lib Dem incumbents to beat off strong challenges, and could make a big difference again.
Tactical voting is not confined to the major parties: there are Green, BNP and Ukip votes up for grabs as well. According to recent polls, supporters of these three parties may account for 10 per cent of the vote or more, so there is a potentially large group of voters who could move one way or the other. Just before the European parliamentary elections in June 2009, YouGov put its forced choice question to a sample of 7,500 supporters of the three smaller parties. Labour was ahead among the Greens, but only by 6 per cent.
The bulk of BNP and Ukip voters preferred the Conservatives: 70 per cent of the Ukip segment chose that option, while just 12 per cent opted for Labour. In the marginals, Ukip supporters in particular could be crucial, and are probably Cameron's biggest hope for an overall majority.
Follow the leader
The polling gap between Labour and the Tories is getting smaller. If the Conservatives hoped their ratings would benefit from the British Airways strike and Labour's links with Unite, they have been disappointed.
But the government's recent problems have had an impact on the leader's approval ratings. In previous elections, notably in 1992, these have proved to be a more accurate predictor of outcomes than polls of voting intentions. The latest YouGov figures put Cameron's net rating at 10 points. Gordon Brown's is at -28 points, which is a very big gap to close.
Mike Smithson is the editor of politicalbetting.com