A marginal seat is one that is held by the incumbent with a small majority. There is no precise percentage or winning margin to which this aligns but a 10% margin would need only a swing of 5% to the rival party to take it.
Marginal seats are important as they are where the outcome of elections are decided and they often attract disproportionate attention, and funding, during an election campaign.
Historically, only a minority of seats in UK Westminster constituencies are marginal. However, in the 2017 general election, 97 of the 650 constituencies were won by a margin of 5% of the vote or less – this was a sharp increase in the number of marginal seats compared with 2015, when 56 seats were won by a less than 5% margin, although only slightly more than the 91 seats in 2010.
Interestingly, under the Alternative Vote system marginal seats would have a yet greater impact; their influence can only be eliminated via a form of proportional representation. In spite of this, concerns about the disproportionate impact of marginal seats formed a major part of arguments against FPTP in the referendum campaign of 2011, during which the following comments were made:
“First Past the Post is broken. The last election was decided by fewer than 460,000 of voters — just 1.6% of the electorate. No wonder people feel so remote from politics and feel it doesn't address their everyday concerns. It is time for change and that is why I am backing a Yes vote."
"Research has shown that the result of the last election was decided by fewer than 500,000 votes in a handful of constituencies that, by mathematical accident, happened to be marginal. That's out of nearly 40 million eligible voters. That means that only one in every 80 voters actually mattered last year."
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