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All it takes is a few polls. Over the past couple of weeks this race has shifted. In recent months the polls have been close but Labour have led most of them: in the first six weeks of 2015 they led two-thirds of polls.

The Tories occasionally took a lead in a lone poll (and often a sizeable lead), but that would soon be watered down by a series of Labour leads.

But the Tories now lead by 2.1 points in May2015’s 5-day Poll of Polls. That takes into account the past six polls, with more weight on more recent ones. So three strong Tory polls are doing a lot of work: last night’s from YouGov (Tories +2); Monday night’s from YouGov (Tories +4); and Monday’s by Ashcroft (also Tories +4).

YouGov have now put the Tories ahead in their past three polls, five of their past seven, and six of their past eleven. Ashcroft has put them ahead twice in a row.

Our 5-day Poll of Polls takes into account the past six polls.

As a result, all the election forecasts – ours, Election Forecast’s, Ladbrokes’, the Guardian’s – have updated and now put the Tories anywhere from 13 (the Guardian) to 28 seats ahead (Election Forecast). We put them ahead by 27, and Ladbrokes by 20. On average the Tories now lead by 22 seats.

A few weeks ago most forecasters showed Labour was likely to be the largest party – especially ‘nowcasters’ like us and the Guardian, who don’t peg current polls to 2010 results.

The arithmetic has changed.

How have the numbers changed? Because the only thing that really changes this race are swings between Labour and the Tories. This race will be decided in places like High Peak, Stevenage, Dover and Morecambe – classic Tory-Labour marginals.

Recent polls have showed a weaker Tory-to-Labour swing than polls did in early 2015, so the number of Tory-held seats we’re predicting to go to Labour has fallen. Because of this and all the seats they’re set to lose to the SNP, Labour are now projected to win fewer than 10 extra seats on the 258 they won in 2010.

At this rate, despite five years of campaigning, Labour are barely going to win more seats than Brown did in 2010, when he won 29 per cent of the vote.

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It all comes back to the Labour-Tory marginals. As the polls swing around, the way forecasters treat these seats will change too, so these forecasters could still swing back in Labour’s favour.

As for debates over how many seats the SNP will win, they don’t actually change the make-up of the next Parliament: both Labour and the SNP will vote against a Tory-led government. And neither Ukip or the Greens are set to win enough seats to affect things directly.

At this rate Labour are barely going to win more seats than Brown did in 2010.

The Lib Dems matter, but most of their battles are with the Tories in areas like the south-west, and if the two parties are going to form another coalition they’re fighting a zero-sum game. Any Tory or Lib Dem gains come at the expense of one another.

If you forget the possibility of a Lab-Lib deal for a moment – which seems unlikely if Nick Clegg retains his seat and given what type of people still support the Lib Dems – there are, crudely, two election outcomes: another Tory-Lib Dem pact, possibly propped up by Ukip and the DUP; or a Lab-SNP pact, helped by a few seats from the SDLP and Greens.

The four forecasts updated today now collectively give the Tories 286 seats and Labour 264. The Lib Dems have 26 and SNP 48. So:

Labour plus SNP = 312

Tories plus Lib Dems = 312

In practice, 323 is a majority. The DUP are set to win 9 seats and Ukip 3. Together they could take a Tory-Lib Dem coalition over the line.

Under this scenario, Labour and the SNP wouldn’t be able to vote such a government down.