1966 Carmarthen by-election in the context of "Welsh Labour"

Plaid Cymru (English: /plaɪd ˈkʌmri/ plyde KUM-ree, Welsh: [plaid ˈkəmrɨ]; lit. 'Party of Wales', officially Plaid Cymru – the Party of Wales, and often referred to simply as Plaid) is a centre-left, Welsh nationalist political party in Wales, committed to Welsh independence from the United Kingdom. It campaigns on a platform of social democracy and civic nationalism. The party is a supporter of the European Union and is a member of the European Free Alliance (EFA). The party holds 4 of 32 Welsh seats in the UK House of Commons, 14 of 60 seats in the Senedd, and 200 of 1,234 principal local authority councillors. Plaid was formed in 1925 under the name Plaid Genedlaethol Cymru ('the national party of Wales') and Gwynfor Evans won the first Westminster seat for the party at the 1966 Carmarthen by-election.

In 1999 (in the first devolved Welsh Assembly election), Plaid Cymru gained considerable ground in traditionally Labour heartlands. These breakthroughs were part of the intentional aim to win more seats in the Welsh valleys and North East Wales, which continues to be an ambition today. The party have mostly been in opposition in the Senedd. Although under the leadership of Ieuan Wyn Jones, the party was part of a coalition as a junior partner with Welsh Labour (See: One Wales, Morgan and Jones governments) between 2007 and 2011. Wyn Jones became the deputy First Minister and Minister for the Economy and Transport, other Plaid Cymru Assembly members were also part of the cabinets.

The February 1974 United Kingdom general election was held on Thursday 28 February 1974. The Labour Party, led by former Prime Minister Harold Wilson, gained 14 seats (301 total) but was seventeen short of an overall majority. The Conservative Party, led by Prime Minister Edward Heath, lost 28 seats (though it polled a higher share of the vote than Labour). That resulted in a hung parliament, the first since 1929. Heath sought a coalition with the Liberals, but the two parties failed to come to an agreement and so Wilson became prime minister for a second time, his first with a minority government. Wilson called another early election in September, which was held in October and resulted in a Labour majority. The February election was also the first general election to be held with the United Kingdom as a member state of the European Communities (EC), which was widely known as the "Common Market".

Its results saw Northern Ireland diverging heavily from the rest of the United Kingdom, with all twelve candidates elected being from local parties (eleven of them representing unionist parties) after the decision of the Ulster Unionists to withdraw support from the Conservative Party in protest over the Sunningdale Agreement. The Scottish National Party achieved significant success at the election by increasing its share of the popular vote in Scotland from 11% to 22%, and its number of MPs from one to seven. Plaid Cymru also succeeded for the first time in getting candidates elected at a general election in Wales (its sole previous seat was won at a by-election in 1966).