07/06/2017 12:55 BST | Updated 07/06/2017 12:55 BST

After An Extraordinary Election, Watch For The 3Ts On Thursday Night

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A very unusual election is drawing to a close. The campaign has been unconventional in that normal issues, such as the economy, have hardly featured. Instead, a quasi-presidential contest has been concentrated on supposed competence to govern. Even the one issue that has regularly and tragically intruded on the campaign, terrorism, has featured largely in this context, while the merits of actual counter-terrorism policies have hardly been broached, let alone debated.

This presidential-style approach was very much the choice of the Tories and supposed to play to the strengths of their 'strong and stable' candidate. As long as voters did not question her competence or Jeremy Corbyn's unfitness to govern this seemed a risk-free strategy. Things started to go wrong, however, from the moment that the leaked Labour manifesto switched attention from May onto Corbyn. The Tories then compounded this by overstated attacks on the Labour leader. Not only did this undermine their own electoral strategy, but they allowed Corbyn to gain attention and traction, particularly by failing to obligingly give credibility to the Tory propaganda. The stridency of their attacks also drew increasing attention to May's own dubious record in areas such as policing and counter-terrorism. Many of the polls still suggest that she will nonetheless triumph on 8 June, but despite rather than because of her campaign.

The polls, meanwhile have been another conspicuous feature of this unusual election. They have been more volatile than usual, while the spread between the various polling organisations has been phenomenal. This partly reflects changes in methodology after pollsters under-called the Tory vote in 2015. But their main problem in calculating the likely vote and seat share come 9 June has been the 3Ts: turnout, territory and tactical voting.

Pollsters adjust for past voting record, including propensity to vote. Differential turnout by older voters in 2015 strongly favoured the Tories. Many pundits are assuming that this will again see the Tories coast to victory. However, the Lynton Crosby dog-whistle tactics of 2015, scaring the voters about Alex Salmond being the backseat driver of a Miliband government have no obvious analogue now. Instead, the Tories' lacklustre campaign seems to have dimmed the enthusiasm even of many of their elderly voters. Most pollsters are assuming that young voters will turnout more than in 2015, but they are not agreed on how much more. I suspect that they should also be dubious about whether older ones will turnout in the same numbers again.

The other imponderable is how much of a purple firewall of ex-Ukip voters do the Tories have? It is noticeable that Ukip was polling in double-figures almost until the May local elections, then collapsed. Did the pollsters adjust too late, or too drastically? Will most of these defectors stay with the Tories now that their former party is denouncing 'weak and wobbly' Theresa? Indeed, given that 15% of the Ukip voters in 2015 apparently were previous non-voters, can we be certain that they will actually turnout this time?

It is, of course, not just a matter of if they turnout, but where. Britain has not had a uniform swing election for years and 2017 is unlikely to buck that trend. Differential turnout, however, will only matter where it can swing outcomes. So the mobilisation of younger voters may not greatly change results if they are largely concentrated in urban settings. Some polls have suggested that this could result in some Tory losses in London, but there are few such seats elsewhere in the country. Tory strategists will be hoping that ex-Ukip support will more than compensate in the disaffected towns of the North and Midlands. Here two factors are likely to be significant. One is incumbency, with polls showing that sitting Labour MPs benefit to varying degrees from a personal vote. The other is the third of the 3Ts, tactical voting.

There has certainly been lots of effort to encourage tactical voting. There is also some polling evidence to suggest, for instance, that Liberal Democrat support in Labour marginals has more than halved in favour of tactical voting against the Tories. This might help to explain the seemingly poor polling record of the Liberal Democrats during the campaign. It does not necessarily mean that they will fail to get their vote out in seats that they hold or at least some of those they are targeting.

The result of the 3Ts is an election which is much more difficult than most to call. The effect of these imponderables is that some recent polls have suggested spreads of more than 80 seats in terms of likely outcome for the Tories. Of course, it could still all turn out that election night ends up as the boring triumph that Theresa May initially hoped for. However, if the 3Ts really play a part then we could all be in for an evening of exciting surprises.