Corby by-election: 'Weathervane' seat is key to the parties' hopes of an outright victory in 2015

Although voters in every part of England and Wales outside London have the chance to go to the polls tomorrow to elect 41 police and crime commissioners, Labour and Conservative strategists will privately be more interested in the parliamentary by-election in Corby.

The former steel town and its surrounding rural areas make up a classic “weathervane” seat which tends to elect an MP from the party which wins power at Westminster. Labour captured it from the Tories in its 1997 landslide and Louise Mensch, the chick-lit author regained it in 2010.

It was the 29th most marginal Tory-held seat, meaning it was in line for special funding and a full-time Tory official, under the party’s “40-40” battle plan to win an overall majority at the 2015 election. Drawn up by Grant Shapps, the party chairman, it will concentrate resources on 40 winnable and 40 Tory-held constituencies.

If, as widely expected, Labour regains Corby today’s, the seat would switch from than “must hold” to the “must gain” column on the Shapps hitlist. “In the long run, Corby will matter more than the police results,” one Labour MP said. “It is seats like this that will decide the next election.”

Labour is quietly confident but playing down expectations of a big majority. Although Ms Mensch won a majority of 1,895 in 2010, Labour campaigners insist a majority of over 1,000 would be good because the turnout will be much lower than at a general election.

Labour officials dismiss as “ridiculous” a survey in Corby commissioned by Lord Ashcroft, the former Tory deputy chairman, suggesting that Labour enjoys a huge 22-point lead over the Conservatives.

In a leaked email sent to Tory MPs, their by-election campaign manager Michael Fabricant tried to raise spirits by claiming the party had “more pledges” than Labour and saying the seat is “not Labour territory.”

However, the Tories are braced for a bloody nose, not least because voters often tend to punish parties which cause unnecessary by-elections. Ms Mensch angered the Tory high command by stepping down halfway through the five-year parliament so she could spend more time with her New York-based husband Peter, manager of the rock bands Metallica and the Red Hot Chili Peppers.

Votes in Corby will not be counted until Friday with the result due early in the afternoon.

Parliamentary by-elections also take place tomorrow in two safe Labour seats - Cardiff South and Penarth and Manchester Central.