When I suggested, following Louise Mensch's resignation, that Labour would walk the Corby by-election, the party's deputy chair Tom Watson cast doubt on my prediction, insisting that "Corby will be a very tough fight" (oddly, his tweet seems to have vanished).
But Watson needn't have managed expectations. A poll of 1,500 Corby voters by Lord Ashcroft for ConservativeHome gives Labour a 15-point lead over the Conservatives, with Miliband's party on 52% and Cameron's on 37%. The Lib Dems are on just 7% and could yet be caught by UKIP. The Tories currently have a majority of 1,951 (3.6%) after winning the seat, which Labour held from 1997 until 2010, at the last election.
Miliband has won five by-elections since becoming Labour leader. with Bradford West the only blot on his record, but Corby would be his first gain. The evidence that the Tories are in retreat in those marginals that they won in 2010 is confirmation, if needed, that they will struggle to avoid defeat at the next election.