As a bellweather constituency which consistently returns an MP of the governing party, a by-election in Corby would generate intense scrutiny at any time.

The contest in November resulting from the unexpected resignation of high-profile Tory backbencher Louise Mensch will, however, provide particularly rich pickings for the political soothsayers. As the first seat that either Conservative or Liberal Democrat parties have defended since forming their Coalition in 2010 it comes when the separate parties are increasingly focused on their differences. At the vulnerable mid-point of the parliamentary term, an anti-Government swing is to be expected and with the economy in the longest double-dip recession on record and the forecasts increasingly gloomy, Labour must be on course to regain the seat.

Nevertheless, the Corby and East Northamptonshire by-election will amount to more than a simple political sounding board. The constituency is marginal because it contains both the solidly Labour town of Corby, which is matched in voting power by the equally stalwart Tory territory in the rural villages of East Northamptonshire.

As the outcome of the 2010 General Election made clear, political allegiances are increasingly fluid and more likely to depend on parties' policies on specific issues and how their leaders are perceived. That will be true of this by-election which will be viewed not only by the party machines but by the voters as a poll on the competence of the Government.

Mrs Mensch herself is an illustration of the importance of a candidate having an attractive image and high recognition factor. She was parachuted into Corby as a Tory A-list candidate, a group hand-picked to personify the message that the party had modernised and was no longer the preserve of middle-aged, white, heterosexual males.

This was crucial to capturing the support of Middle England voters who had voted for Tony Blair's New Labour in 1997. Among them was the future Tory MP for Corby, who had joined the Labour Party in 1996. Two years ago she won the seat from Labour's Phil Hope, whose 13-year tenure was tainted by the expenses scandal. Whoever the Tories select as the candidate this time around will have a much tougher task.

Given the unpopularity of the Coalition's deficit reduction policies and the particular despair of LibDem voters at the failure of their party to secure backing for policies such as Lords reform, Labour must pick up their votes in this seat if it is to win in 2015.

Ed Miliband seized the initiative yesterday in visiting the constituency with a pledge to make unemployment, especially among young people, the key issue in the campaign. It is one close to the hearts of the people of Corby whose economy was devastated when the steelworks closed in 1980 with the loss of 11,000 jobs. But Mr Miliband must produce policies if he is to convince the voters of this marginal constituency that Labour offers a credible alternative government.