1885–1922

The constituency was a predominantly Nationalist area in 1918. The Unionists had significant but minority support. The Sinn Féin candidate polled poorly, probably due to the limited electoral pact to avoid seriously splitting the anti-unionist vote in seats the unionist candidate might have otherwise won.

The First Dáil

Sinn Féin contested the general election of 1918 on a platform that instead of taking up any seats they won in the United Kingdom Parliament, they would establish a revolutionary assembly in Dublin. In republican theory, every MP elected in Ireland was a potential Deputy to this assembly. In practice, only the Sinn Féin members accepted the offer and their candidate Éamon de Valera only received 0.2% of the votes in South Down, while being elected unopposed for East Clare and East Mayo; the nationalist Jeremiah McVeagh, elected as IPP MP for South Down, did not participate in the First Dáil.

In 1921, Sinn Féin decided to use the UK authorised elections for the Northern Ireland House of Commons and the House of Commons of Southern Ireland as a poll for the Irish Republic's Second Dáil. This area, in republican theory, was incorporated in a potential eight-member Dáil constituency of Down.

1950–present

When initially created, this 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 in 1987, he narrowly lost to Eddie McGrady of the Social Democratic and Labour Party, 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.

Elections in the 2020s

Elections in the 2010s

This seat saw the largest decrease in vote share for the SDLP at the 2019 general election.^[10]

Elections in the 2000s

Elections in the 1990s

Elections in the 1980s

Elections in the 1970s

Elections in the 1960s

Elections in the 1950s

Elections in the 1910s

Elections in the 1900s

Elections in the 1890s

Elections in the 1880s