I had a look at the local election results that overlap the parliamentary borough of Crewe and Nantwich, for each year since 2005. The results are encouraging ahead of the by-election.
In 2005 the Tories lost by 7,000 votes with 32.6% to Labour's 48.4%. In the Cheshire County Council elections held at the same time Conservatives got nearly exactly the same number of votes as in the Westminster election, but Labour did not. In fact, in those County Council elections Labour got only 35.1% or 5600 votes less, with most of those people voting for local independents.
Fast forward to the new unitary authority elections held on May 1st of this year. In the wards that are in the parliamentary constituency (more or less) Conservatives got 39.3% to Labour's 26.2%. This is obviously encouraging, but we should add back in many of those Labour voters who voted an independent in the local elections. Assuming the difference is the same as it was in 2005, about 13% should be added back onto Labour's May 1st result - which is 39.2% - a dead tie with this year's Tory vote. That's how a 7000 majority can vanish very quickly.
Another factor in favour of the Conservative candidate is turnout. Looking at all the elections in the constituency since 2005 there is a very strong correlation between low turnout and Conservative vote (the correlation coefficient is -.985 for all you math geeks out there). So the low turnout in a by-election (somewhere in the order of 35%) should help the chances of a Conservative victory.
