The death of Labour veteran Gwyneth Dunwoody brings a by-election of huge significance to all three major party leaders.

For embattled Gordon Brown the contest in her Crewe and Nantwich constituency will come with his popularity at a low ebb amid crises in the economy and the housing market.

The 8.2 % mid term swing needed for a Tory gain might be expected to be within David Cameron's reach but his party's record in recent Commons by-elections has been abysmal, with a third place at Southall last year and near defeat at up market Bromley and Chislehurst in 2006.

Nick Clegg's Liberal Democrats, who came a fairly strong third in the constituency last time, will have the challenge of matching their impressive by-election triumphs of the past.

A Press Association survey of the latest Crewe and Nantwich Borough elections suggest Tories are at least well-placed at the start.

Analysis of the 2007 polls - using 2006 results for five wards that were not up last year - give totals of Conservative 11,195, Labour 7,439, Lib Dem 2,643 and others 2,907.

One ward was not contested by Labour and figures are not included for another, small, Tory one last fought in 2004.

Liberal Democrats had candidates in less than half the wards and the figures are not an indicator of their true strength.

The seat, in its current and previous forms, has been Labour since the Second World War. The nearest Mrs Dunwoody came to defeat was when she squeaked home by just 290 in Margaret Thatcher's high tide year of 1983.

The constituency's industrial base is centred on Crewe while nearby Nantwich is a traditional market town.

The rural hamlet of Crewe saw its Cranford-like solitude overwhelmed in 1843 when it became the site of West Coast main line's main railway works, enabling it to rapidly outstrip its more historic and rural neighbour.

It was the junction of trunk lines to Liverpool and Scotland and to Manchester and spawned the poem: Oh Mr Porter, what can I do?

I wanted to go to Birmingham.

But they took me on to Crewe ...

The rail works, now owned by Bombardier, are still operating, alongside other industries which include the Bentley car factory which had also produced Rolls-Royces until 2002.

The constituency is also within commuting distance of Manchester and Merseyside.

A possible by-election issue is the current local government shake-up in Cheshire, creating two new all-purpose unitary authorities and bringing the axe down on Crewe and Nantwich Borough Council next year.

The proposals were fiercely criticised by Mrs Dunwoody earlier this year in one of her last Commons speeches.

Labour's business managers face a tough choice in selecting a by-election date.

They could go for late May, denying opponents time to build up a campaign, or pick a day in July, hoping memories of any poor result will fade over Parliament's long summer break.

:: Crewe and Nantwich General Election Result May 2005: Dunwoody (Lab) 21,240 (48.8%); Moore-Dutton (C) 14,162 (32.6%); Roberts (Lib Dem) 8,083 (18.6%). Majority 7.078.