After the return of economic growth and the narrowing of Labour's poll lead prompted some Tories to talk of a "glide path to victory", Lord Ashcroft's marginals poll  has provided a much-needed dose of realism. The survey of the Conservatives' 40 most vulnerable constituencies shows that the defection of Tory supporters to UKIP means that Labour now enjoys a 14-point lead in the 32 seats where it is in second place, compared to a national lead of just five in Ashcroft's poll. In short, the party is gaining support where it matters most. Labour is on 43% (down one since 2011), the Tories are on 29% (down six), UKIP is on 11% and the Lib Dems are on 8%. That lead is large enough for Miliband's party to win the 32 most competitive Con-Lab marginals and a further 66 off the Tories if the swing is replicated elsewhere, putting it on course for a comfortable majority.
But it isn't just Labour and UKIP that have cause to be cheerful; there's also some rare good news for the Lib Dems. In the eight most marginal Con-Lib Dem seats, Nick Clegg's party is just three points behind David Cameron's, with a swing of only 0.5% to the Tories since 2010. The Conservatives are on 32%, with the Lib Dems on 29%, Labour on 18% and UKIP on 12%. For the Lib Dems, it is further evidence that their vote is holding up where they are competitive. Rather than merely defending their existing 57 seats, the surge of UKIP (which draws around 60% of its support from 2010 Tories) means that the Lib Dems could yet hope to dislodge the Tories in seats where they are vulnerable.
The poll will gladden Labour hearts and darken Tory ones but it's important to remember, as Ashcroft says, that it is "a snapshot", not a prediction. It tells us what would happen were a general election held today, not what is likely to happen in 2015. Governments invariably gain support in the run-up to a general election as voters stop treating opinion polls as a referendum on the government (2010 was typical of this), so Labour needs a large cushion of support to be confident of victory. A similar poll conducted by PoliticsHome in September 2008  suggested the Conservatives would win a landslide majority of 146 seats, while another, carried out in October 2009 , pointed to a Tory majority of 70. Just seven months later, Cameron was left with no majority at all. In other words, 18 months out from the general election, only the most optimistic Labourite or the most pessimistic Tory would treat this poll as a reliable indicator of the result.