It’s not polarisation that makes houses of US Congress move differently

Those who aren’t American – or don’t care about American politics – might wonder why the November 6 mid-terms are expected to result in the two house chambers moving in different directions.

That’s because of the electoral map.

In the House of Representatives, where all 435 seats are up for grabs, Democrats have momentum. They’re hoping to take control of the House and the polls suggest they might just do that. As The Economist ’s statistical model, recently put it, the Democrats have an 86 per cent chance of gaining a majority.

The US Senate, however, is a different matter. Only 35 of its 100 seats are being contested this year. Only nine of those seats are held by Republicans, which means 26 Democrat Senators are defending their turf.

For Democrats, the electoral map is yet another unpleasant reality for 2018.