Barring some dramatic upset, Labour will win today's Corby by-election (triggered by the resignation of Louise Mensch), gaining a seat from the Conservatives for the first time in this parliament. But winning alone isn't enough. A slender victory would prompt fears that the party still isn't where it needs to be if it's to stand a chance of winning the next general election. Thus, while Labour requires a swing (the rise in one party's vote added to the fall in another's and divided by two) of just two per cent to overturn the Tories' majority of 1,951, it needs a swing of at least five per cent to put it on course to win a national majority in 2015 and a swing of eight per cent to justify its average opinion poll lead of nine points. The latter would see it take the seat with a majority of around 12 per cent. 

To win Corby: Labour needs a swing of two per cent.

To win a majority in 2015: Labour needs a swing of five per cent to put it on course to win a majority at the next general election.

To match its national poll lead: Labour needs a swing of eight per cent to match its average national poll lead of nine points.

Polling by Lord Ashcroft in the constituency has suggested that Labour will easily exceed this benchmark. His final survey gave the party a lead of 22 points (with Labour on 54 per cent and the Tories on 32 per cent) and showed a swing of 12.5 per cent from the Tories to Labour.

Labour, naturally, has downplayed Ashcroft's findings, insisting that Corby will be "a very tough fight" in an attempt to ensure its supporters turn out. Ed Miliband reminded party workers last week that the party thought it had won in 1992 but ended up losing to the Tories by 342 votes. While history isn't likely to repeat itself today, anything less than a resounding victory in this mid-term electoral test will force Labour to ask why it isn't doing better.

Update: As the commenter below points out, there are two other by-elections today, in Manchester Central and Cardiff South and Penarth. Labour currently holds both seats by majorities of 10,430 and 4,709 respectively and is expected to retain them.

In addition, the first ever police and crime commissioner elections are taking place in 41 police authority areas in England and Wales. You can read my guide to the elections here.