Voters in the constituency of Corby are heading to the polls today to elect a new MP.

The by-election follows the resignation of former Tory MP Louise Mensch who stood down in August citing family reasons. She has moved to New York to be with her husband.

So why is Corby so important?

This is the first by-election where the Conservatives are defending a seat since David Cameron became Prime Minister in May 2010 and opinion polls suggest it will be difficult for them to hold on.

Corby is a marginal seat, the Tory majority in the General election was just 1951 votes. Since the constituency was formed in 1983 voters in Corby have always elected a MP from the same party of the Prime Minister of the day .

This has been a busy and very long by election campaign, Labour leader Ed Miliband was in the constituency on August 9th to kick off Labours campaign. Harriet Harman, Nick Clegg, Eric Pickles and Theresa May are just some of the big name politicians who've joined the campaign trail in Northamptonshire. And MPs from across the East have taken party supporters from their constituencies to help out their candidates.

The constituency is named Corby but it encompasses not just the more Labour leaning town of Corby but very many villages in East Northamptonshire where many voters traditionally back the Conservatives. Many MPs say crucial to both Labour and the Tories is making sure their supporters go out and vote, rather than winning over those who've not supported them in the past.

It may prove difficult to persuade voters to the polls on a cold dark November evening.