Most elections bring some first-time breakthroughs for one party or another. The 2017 general election was relatively unusual in seeing both the main parties conquering some entirely new territory: for instance, Labour gained Canterbury and the Conservatives took Mansfield. When something like this happens, it suggests not only that something interesting is going on in the relationship between the parties and their traditional bases of support, but also that demographic change has brought some traditional strongholds to the point of marginality where they are at risk of switching sides.
Demographic change has always been affecting elections. Sometimes it is obvious and straightforward; the Conservatives lost the 1966 election badly but still held on to both Birmingham Handsworth and Manchester Moss Side—but could not do so for much longer because of ethnic change. Lichfield & Tamworth was Labour in 1959 but is now the basis for two pretty solid Conservative seats. Sometimes it is more complex, particularly given the way the housing market operates in big cities. To take one example, the Roehampton estate in Putney. Its construction helped to tip the seat to Labour in 1964 by increasing the working class population in this suburban seat; by the 1980s and 1990s the right to buy had converted many of the tenants into grateful Conservative voters. Another turn of the wheel since, and the flats are being let out to disgruntled renters on the private market and the ward—and nearly the constituency—has reverted sharply to Labour.
There is, at the moment, an asymmetrical relationship between demographic change and the direction of travel in electoral terms. Rapid local changes in the composition of the electorate will usually revolve around groups which are increasing in size, and these tend to favour Labour: black and minority ethnic communities, young people and students, graduates, private renters. Places that are trending to the Conservatives will often be those precisely where demographic change is working most slowly: rural communities, affluent anti-development suburbs, and older white working class towns and villages. The median age of the population in Manchester and Newcastle has dropped because of visible influxes of young people. The median age in, say, Forest of Dean or Tamworth has gone up because of more gradual differential migration as industries decline or the existing inhabitants have pulled up the housing ladder behind…