Psephologist Robert Waller analysed the marginal seats and boundary changes in every UK region for Total Politics in a long-running series from December 2008 onwards. With the election now just over two weeks away, we will look back at his findings in the run-up to the election. Today we look at North West England - an area around Lancashire that has more critical marginals than anywhere else in the country. Read the full article here.

The north west of England has always been a key region in deciding the people of Britain's choice of government. It comprises the metropolitan boroughs within Greater Manchester and Merseyside, and the administratively separate county of Lancashire (a geographical entity which historically included the first two areas).

The next general election will prove no exception. David Cameron's Conservatives will have to demonstrate an ability to make a very large number of gains in the north of England as well as the midlands and south; and there are indeed a plethora of opportunities in the tight marginals of the north west.

Two features of the pattern of contest here may be picked out immediately. One is that there are actually fewer super-close seats based on 2005 notional results than might be expected. For example, the 'easiest' Tory gain, Bury North, ranks only at number 33 on the national Conservative target list of current Labour seats.

The most important constituencies are those which need to fall in order to make the Conservatives the largest party, or give them an overall majority. There are many in these key categories in the north west - in fact nearly 20, if potential gains from the Liberal Democrats are also counted. This number is not exceeded in any other region.

Secondly, there is a myth that the Tories will need to make substantial advances in the big northern cities and metropolitan boroughs if they are to win the general election. This is not true. The majority of their vital targets even within this predominantly urban region are not in Merseyside or Greater Manchester, but in the shire county of Lancashire. This applies both to seats needed to become the largest party, and to those needed to go on and secure an overall majority. Indeed, well over half of the 16 seats now to be allocated to Lancashire are critical marginals - the highest proportion of any county.

Pendle is named after the great and mysterious hill, historically known for its witches, that looms above communities including Nelson & Colne; Gordon Prentice may have to resort to sorcery to ward off the challenge of the Tory candidate Andrew Stephenson, who needs a swing of less than three per cent from 2005.

In Rossendale & Darwen, almost all the wards which make up the seat are marginal, and the Conservatives won almost all of them in 2008. Hyndburn, based on Accrington and containing the very heart of the famous Lancashire cricket league, needs a larger swing of seven per cent, but the Tories will need to take seats like this for the overall majority that many opinion polls suggest is within their grasp.

South Ribble, in the north of Lancashire, is a classic swing marginal likely to be won by whichever party comes out of the election as the largest. It is centred on Leyland (as in commercial road vehicles) where David Borrow defends a majority of just over 2,000.

Harder for the Tories will be Morecambe & Lunesdale, which despite the image of the seaside and its attractive rural hinterland remains almost drawn to give Labour a good chance. Not only is Morecambe itself battling to reverse a long period of depression, but the seat includes the most deprived suburb of Lancaster, Skerton, as it is located on the west bank of the Lune.

The Conservatives have no councillors in the city of Manchester, and no chance of a parliamentary gain there either. However, they do have winnable targets within the other boroughs in Greater Manchester. A swing of only 2.6 per cent would be enough to take Bury North, which consists of the home of black pudding itself together with the small town of Ramsbottom, which is a more up-market residential area than the name might imply.

However, the key to whether the Tories can take power at Westminster probably lies most of all in Bolton, a town known for producing the comedian Peter Kay of Phoenix Nights fame, as well as a ring-full of professional wrestlers. It also harbours two key marginal seats, North East and West (where Ruth Kelly is retiring despite favourable boundary changes), each requiring a swing of exactly six per cent. If the Conservatives can win both of these, it will be no laughing matter for Labour, who will be in a hammer-lock or full nelson which will probably force them to submit - not just in Bolton, but in the grapple for government itself.

Robert Waller is co-author of The Almanac of British Politics (Routledge).

Tomorrow: Wessex