His shooting spree lasted an hour and 13 minutes, as he methodically stalked and killed many of the 600 up - and - coming leaders of Labour, Norway's dominant political party, which he blamed for the rise of multiculturalism. (businessinsider.com)
But in electoral politics you are forced to choose between the two dominant political parties. (firstthings.com)
Duverger's law says that this will tend to produce two dominant political parties, the winner and the loser. (politics.stackexchange.com)