The constituency was a predominantly Nationalist area prior to 1922. Although Sinn Féin leader Éamon de Valera was on the ballot at the 1918 general election, he did not campaign, as part of a pact with the Nationalist Party.
When it was recreated in 1950, the seat had a clear unionist majority, albeit with a strong nationalist minority. However boundary changes, which have wrapped it closer around nationalist heartlands near Downpatrick and the Mournes, have transformed South Down into a safe nationalist seat.
The House of Commons seat was consistently held by the Ulster Unionist Party from its creation until 1987. In the October 1974 general election the former Conservative MP Enoch Powell defended the seat for the UUP, representing a coup for them as they gained the support of a high-profile English politician, offering them a spokesperson to the United Kingdom as a whole.
Powell advocated a policy of integration for Northern Ireland whereby all forms of devolution would be wound up and the province governed as an integral part of the United Kingdom. As part of this, he campaigned for the province to have the same ratio of MPs to population as in the rest of the United Kingdom, rather than fewer, which had previously been justified due to the existence of the devolved Stormont Parliament. Powell was successful in this, but a side effect was that in his own constituency, a significant block of unionist voters were removed, resulting in a nationalist majority. Powell managed to survive for two election cycles due to a split nationalist vote, but at the 1987 general election, he narrowly lost to Eddie McGrady of the SDLP, who held the seat until he retired in 2010.
Since then, the unionist vote has declined further due to boundary changes, which excluded mainly unionist Dromore and Saintfield, and a trend for many unionists to tactically vote for the SDLP at Westminster elections to avoid the seat falling to Sinn Féin. However, in 2017, Sinn Féin gained the constituency for the first time with Chris Hazzard defeating former leader of the SDLP Margaret Ritchie as part of the SDLP's parliamentary wipeout at that year's snap general election.
The winning vote share in 2019 was the smallest of the 650 nationwide; it was just under 1⁄3 of the total votes that were cast.
Under the Redistribution of Seats (Ireland) Act 1918, Newry ceased to exist as a parliamentary borough, and the parliamentary county gained the additional division of Mid Down.[2] At the 1918 general election, Sinn Féin contested on an election manifesto with an abstentionist pledge that instead of taking its seats at Westminster, it would establish an assembly in Dublin. All MPs elected to Irish seats were invited to participate in the First Dáil convened in January 1919, but no members outside of Sinn Féin did so.[3]
Under the Representation of the People Act 1948, all two-member constituencies were divided. Down was divided into the county constituencies of North Down and South Down. The area was reduced in 1983 as part of an expansion of Northern Ireland's constituencies from 12 to 17 with significant parts of the constituency transferred to Upper Bann and Newry and Armagh.
In the district of Down, the wards of Ardglass, Audley's Acre, Ballymaglave, Ballymote, Ballynahinch East, Castlewellan, Cathedral, Crossgar, Donard, Drumaness, Dundrum, Dunmore, Killough, Kilmore, Murlough, Quoile, Seaforde, Shimna, Strangford and Tollymore;
in Banbridge, the wards of Ballyward, Bannside, Katesbridge and Rathfriland; and
in the district of Newry and Mourne, the wards of Annalong, Binnian, Burren and Kilbroney, Clonallan, Derryleckagh, Donaghmore, Kilkeel Central, Kilkeel South, Lisnacree, Mayobridge, Rostrevor, Seaview and Spelga.[12]
2010–2024
In the district of Banbridge, the wards of Ballyward, Bannside, Katesbridge and Rathfriland;
in the district of Down, the wards of Ardglass, Audley’s Acre, Ballymote, Castlewellan, Cathedral, Crossgar, Donard, Drumaness, Dundrum, Dunmore, Killough, Murlough, Quoile, Seaforde, Shimna, Strangford and Tollymore; and
in the district of district of Newry and Mourne, the wards of Annalong, Binnian, Burren and Kilbroney, Clonallan, Derryleckagh, Donaghmore, Kilkeel Central, Kilkeel South, Lisnacree, Mayobridge, Rostrevor, Seaview and Spelga.[13]
2024–
In Armagh City, Banbridge and Craigavon, the part of the Banbridge East ward to the east of the eastern boundary of the 2008 Upper Bann constituency, the part of the Gransha ward to the south of the southern boundary of the 2008 Lagan Valley constituency, and the wards of Loughbrickland ward, and Rathfriland;
and in Newry, Mourne and Down, the wards of the Annalong, Ballydugan ward, the part of the Ballyward ward to the south of the Ballyward split line, Binnian, Burren, Castlewellan, Cathedral, Derryleckagh, Donard, Dundrum, Hilltown, Kilkeel, Knocknashinna, Lecale, Lisnacree, Mayobridge, Murlough, the part of the Quoile ward to the south of the Quoile split line, Rostrevor, the relevant area in the Strangford ward, Tollymore, and Warrenpoint.[14]
