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Two weeks ago, we suggested that Ed Miliband was headed for Number Ten. “It is hard to see how David Cameron can cobble together the 323 seats he will need for a majority,” we argued.

That piece examined the most optimistic seat scenarios for Cameron, and showed how he would struggle to keep power even if they came true. We concluded that Cameron could still win, but his path to victory was clearly more difficult than Miliband’s.

We then introduced what we think is the key graph for this election. It maps all the crucial Tory-Labour marginals. These are the 80 or so seats that the Tories won in 2010 and Labour are trying to win back. We ran through the election’s other 570 seats and showed how if we are right about them, Miliband will only need to win 35 of these Tory seats to dethrone Cameron.

To work out how likely that was, we compared the probability of a Labour victory as suggested by Election Forecast, whose predictions are reproduced by Nate Silver and Newsnight, with the margin of victory in each seat according to our election model, which is based purely on Ashcroft and national polls.

Here’s that graphic, as it appears in this week’s New Statesman. [1] If a seat is in the top right quadrant, we and Election Forecast agree it is likely to vote Labour.

As you can see, 33 seats are in the top right quadrant. In other words, if Miliband wins all of these, he would only need a pair of the many green and light blue seats in the graphic, which we and EF think are very much in play; we either disagree on them (the green seats) or agree that the Tories will just prevail in them (the light blue ones).

But have things now changed? Are 33 seats still in the top right? What’s happened in the green seats, the election’s truly close marginals? And have the numbers changed in other seats – does Miliband still need to win 35 from the Tories?

How have things changed? Does Miliband still need 35 Tory seats?

We’ve re-run the graphic after more than 20 new national polls and about as many new seat polls. And the numbers still look best for Labour.

First, Miliband no longer seems to need as many as 35 of these seats. New Ashcroft polls have given Labour and the SNP double-digit leads in two seats we had assumed Cameron could count on – the Lib Dem seat of Bristol West and the Tories’ sole Scottish seat of Dumfriesshire. (We optimistically assume the Lib Dems, and every individual Lib Dem MP, will back Cameron.)

These Ashcroft polls make sense given national polls, so we have now moved these two seats into Labour’s bloc (we assume SNP MPs will vote for Miliband in a Queen’s Speech, whether they continue to is not our concern here).

And that means Miliband now only needs 33 Tory-held seats.

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So are 33 seats still in the top right of our graphic? Yes. 34 now are. (Click on the graphic for a bigger, zoomable version.)

There hasn’t been much movement overall. National polls have swung very slightly towards the Tories, moving most of the seats leftwards, but the probability of a Labour victory according to EF has, on average, actually risen by nearly 1 per cent.

34 seats are now in the top right, but that’s too binary a conclusion. More accurately, 27 seats look particularly likely to turn Labour. Can you see that there is an upmost upper right quadrant, marked by two thin dotted red lines? That’s an arbitrary box we created for all seats where Labour lead by at least 4 per cent and have at least a two-in-three chance of winning according to EF. There are 27 seats either in or on the edge of this box.

Miliband has to win six of these 16 truly close Tory-Labour seats.

If Labour can win all of these seats, Miliband would be a half a dozen seats away from ‘locking Cameron out’. Can he win another six? The odds seem to be in his favour – there are a dozen seats where Labour appear to be very competitive. They are the highlighted seats in the graphic.

We think these dozen seats will decide the election. If we want to cast the net slightly wider, we can add four seats – Rossendale & Darwen, Pudsey, Stevenage and Crewe & Nantwich – where Labour seem to be slight underdogs but could prevail.

Miliband needs six of these 16 seats. Can he do it? Election Forecast are convinced Labour will win a pair of these 16 – Amber Valley and Lincoln. And the polls suggest Labour are a good few points ahead in another four seats where EF also think they are favourites – Harrow East, Warrington South, Ipswich and Stockton South.

If Labour wins all six of these, Cameron won’t be able to survive even if he wins all the other closest marginals – Finchley & Golders Green; Halesowen & Rowley Regis; Northampton North; Norwich North; Peterborough; and Ealing Central & Acton, as well as Stevenage; Rossendale & Darwen; Pudsey; and Crewe & Nantwich.

It’s clear who should be favoured if we trust the data. It’s also worth noting that Election Forecast assume the Tories out-poll Labour on Thursday by nearly 2 points; that’s why their numbers are slightly less favourable to Labour than ours, which point to a 0.5 point Tory win. Our numbers are based purely on the polls, whereas EF discount the polls in a way that slightly favours the Tories (as history – albeit a fairly limited history – suggests they should).

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We think Miliband remains the most likely post-election PM. But Cameron could certainly win. We think there are four ways he keeps power.

First, the seats could simply break Cameron’s way. Our graphic could be very accurate, but Cameron could just pip Labour in 11 of the 16 key seats we’ve identified, as he must; as well as winning all the seats to the bottom left of the graphic, as we’ve assumed. The numbers could just go the Tories’ way on election night. We are talking about a few extremely close seats that are little more than coin tosses. (Coin tosses that are slightly rigged in Labour’s favour.)

Or second, the polls – and therefore our graphic – could be wrong. In the past fortnight a concerning divide has opened up between the two different types of polling: phone and online. Until a pair of recent ties, six consecutive phone polls had suggested the Tories now lead by more than 3 points. That is a greater lead than EF are predicting, and would shift the seats in our graphic towards the bottom left.

We think there are four ways David Cameron can keep power.

But online polls haven’t picked up such a divide. They still show a tied race, and most polls are online polls, so we and EF still predict quite a close race. If that’s wrong, and the phone polls of the past fortnight are right, the Tories’ odds in these marginal seats are better than we think. To put it another way, Ashcroft’s seat polls could all be systematically slightly wrong, and be showing vote shares that are too low for the Tories.

This is the same dilemma that faced forecasters like Nate Silver during the 2012 US election. Less than a week before polling day in November 2012, national and state polls were telling a slightly different story, in the same way that recent UK phone polls are telling a different story to online and seat polls.

Could “state polls systematically overrate Mr. Obama’s standing?”, Silver wondered five days before the election. “It’s certainly possible. (It keeps me up late at night.) If the polls in states like Ohio and Wisconsin are wrong, then FiveThirtyEight — and all of our competitors that build projections based on state polls — will not have a happy Nov. 6.”

The same will be true for us on Thursday night if polls are wrong, but, as one pollster put it to us last week, “a lot of polls have to be wrong”; Ashcroft’s seat polls fit with the picture of a tied national race shown by dozens of online polls over the past six weeks.

This is the same dilemma that faced forecasters like Nate Silver.

Instead – and this is Cameron’s third way of keeping power – the polls could be right at the moment, but swing the Tories’ way by Thursday. It may seem late for things to change, but Benjamin Netanyahu’s recent re-election was arguably evidence of how a centre-right government can squeeze the vote of a right-wing party just before polling day by stoking fears over statehood.

Fourth, if Cameron’s ‘Netanyahu strategy’ fails; and the phone polls were just an aberration; and things break in Labour’s favour as the odds suggest they should; then Cameron still has one way of keeping power. If he is seen to have won decisively on election night, perhaps some Labour MPs will balk at the chance to unseat him.

The Tories could win nearly a million more votes than Labour, and 20 more seats, and yet still face being voted down by an ‘anti-Tory’ majority. The Times is reporting today that some Labour MPs are concerned by whether they could then legitimately vote Cameron down.

Only 22 per cent of eligible voters voted for Tony Blair in 2005. Few recent Prime Ministers have had much legitimacy. But perhaps a few Labour MPs will choose to abstain when Cameron brings forward a Queen’s Speech. That’s not something we can model.

Mathematically, Miliband is the favourite. In practice, perhaps Cameron can find a way to survive.

A flurry of final polls are due over the next 48 hours. May2015’s final election prediction will run shortly after their release.

[1] By the time our graphic ran in the NS, Miliband’s magic number was down to 34, as we go on to explain.