Complex sentence: If a country becomes polarised, many constituencies will have strong majorities in certain seats, this will mean that marginal seats could be few and far between, making it hard for many seats to change hands when a party's vote drops.
Split into simpler sentences: If a country becomes polarised, many constituencies will have strong majorities in certain seats. This will mean that marginal seats could be few and far between, making it hard for many seats to change hands when a party's vote drops.