What makes the 2015 general election so unusual is that rather than squeezing each other's votes (as is traditional), both the Tories and Labour are focused on squeezing smaller rivals: Ukip in the case of the Tories, and the Greens and the SNP in the case of Labour (with the Farageists also a threat to them). The winner will be the party that best persuades swing voters that they can't afford to let "the real enemy" in.

Labour, as I reported earlier this year, has responded to the Green threat by establishing an electoral unit led by Sadiq Khan. The results have already been seen with Ed Miliband making two major interventions on climate change, a new tone and policy on fracking, and pieces by Khan and Lords chief whip Steve Bassam emphasising how the party has changed since the New Labour era.

Encouragingly for Miliband, we now have evidence that Labour will be able to squeeze some of the Greens' support (some polls have put them as high as 9 per cent). A YouGov poll for the Times's RedBox found that a third of the party's current voters would back Labour at the general election (a similar number to that shown by the party's private polling).

Stephan Shakespeare writes: "Using YouGov’s First Verdict instant polling platform, with a large sample (4,335), I asked: 'Which party would you most like to win in your constituency? Please choose the party you like best, regardless of whether they have a chance of winning​.' I followed up with: 'How do you predict you will actually end up voting? This may not be for your favourite party but for your tactical choice'. There was some movement to and from all parties, but just one example of real significance: a third of the Green vote went to Labour. If we think this will hold, then we should factor in a couple more percentage points for Labour. Of course there may be even more from Ukip back to the Conservatives, but we didn’t see it in this experiment."

If Labour is able to squeeze the Greens and, with a new leader in Scotland, the SNP, it may yet achieve what no Labour opposition has done for at least 50 years and increase its share of the vote in the final six months. Indeed, some in the party view the Green threat as an opportunity. By engaging new voters and non-voters, Labour's rival has made it possible for the party to appeal to people it might otherwise have missed.