In general elections, constituencies where the local result has a habit of matching the national result are known as “bellwethers”.
These are seats that tend to change hands only when the government itself changes hands.
A handful of constituencies have been bellwethers for decades, and as such are watched closely for what they may – or may not – suggest about the overall election result.
The seat with the longest track record of being a bellwether is Dartford.
Since 1964, whichever party wins Dartford has also gone on to form the government.
At this election, Conservative candidate Gareth Johnson is defending a majority in Dartford of 13,186.
For the seat to change hands on December 12, there would have to be a large swing to Labour of 12.2%.
Elsewhere, three seats have been bellwethers at every general election since February 1974: Loughborough, Northampton North and Watford.
All three are on Labour’s target list at this election, and would fall on swings of 4.0%, 1.0% and 1.8% respectively.
Worcester has been a bellwether since 1979. At the 1997 general election, attracting the support of so-called Worcester woman was considered by Labour to be vital for victory nationwide.
At this election, Labour would need a swing of 2.5% to take Worcester, where Conservative candidate Robin Walker is defending a majority of 2,490.
There are also four seats that have been bellwethers since 1983: Amber Valley, Corby, Norwich North and Reading West.
Of these, Norwich North offers Labour the best chance of victory, with a 0.6% swing enough to overturn the Conservative majority of 507.