In 1999 (in the first devolved Welsh Assembly election), Plaid Cymru gained considerable ground in traditionally Labour heartlands.[38] 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.[39] 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.[40] 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.
After losses in the 2011 Assembly elections and dropping down to being the third largest party, Wyn Jones stepped down. He was succeeded by Leanne Wood.[41] In the 2016 Assembly elections Wood managed to win her constituency seat of Rhondda meaning the party gained one seat, and became the official opposition once again, although only for a brief period.[42] In 2018 following internal pressure and a leadership contest, Adam Price defeated Wood and was elected the new leader. Following the 2021 Senedd election Plaid formed a co-operation agreement with the Welsh Labour government.[43] In May 2023 Price resigned as leader following the publication of a report which detailed failings by the party to prevent sexual harassment and bullying. [44] In June 2023 Rhun ap Iorwerth was elected unopposed as leader.[45] The party won the second most seats in Wales in the 2024 general election and won both its target seats. [46]
^"Plaid Cymru". Politics.co.uk. Retrieved 26 December 2022.
^ abSchrijver, Frans (2006). Regionalism After Regionalisation: Spain, France and the United Kingdom (Thesis). Amsterdam University Press. p. 330. hdl:11245/1.288031. ISBN978-90-5629-428-1.
^Siaroff, Alan (2000). Comparative European Party Systems: An Analysis of Parliamentary Elections Since 1945. Garland. p. 467. ISBN978-1-138-88809-8.
^ abElias, Anwen (2006). "From 'full national status' to 'independence' in Europe: The case of Plaid Cymru — the Party of Wales". European Integration and the Nationalities Question. Routledge. p. 194.
^Dunphy, Richard (2004). Contesting capitalism?: Left parties and European integration. Manchester University Press. p. 157. ISBN0-7190-6803-7.
^McEwen, Nicola; Parry, Richard (2005). "Devolution and the preservation of the United Kingdom welfare state". The Territorial Politics of Welfare. Routledge. p. 53.