From Longman Dictionary of Contemporary Englishmarginal seat/constituencymarginal seat/constituencyBritish EnglishPPV a seat in a parliament or similar institution, which can be won or lost by a small number of votes → marginal
Examples from the Corpus
marginal seat/constituency• Another factor not taken into account before the election was the number of expatriate Tory voters registered in marginal constituencies.• Hand in hand with this measure goes an equally bold re-focusing of Labour's strategy concerning marginal constituencies.• She said Darlington had been chosen because it was a marginal constituency.• There is a core vote-a traditional solid Labour support-in every constituency in the land, including marginal seats.• In the 1979 and 1983 elections there were examples of locally popular candidates holding their marginal seats against the national swing.• Mr Devlin's constituency was a marginal seat before Parliament was dissolved last week, having a majority of 774.• One of the country's two most marginal seats, Brecon and Radnor, also declares today.• The Prime Minister rounded off his campaign by visiting two Tory marginal seats in south London. 