Plaid Cymru has traditionally been strongest in the Welsh-speaking areas of Wales, particularly the north; but it was at Carmarthen in west Wales that the party gained its first MP, Gwynfor Evans (then the party's President) being elected to Parliament in a by-election in 1966. He lost the seat to Labour in 1970, but regained it in 1974, during a period when Plaid Cymru was increasingly being seen as an alternative for Labour voters even in the industrial, mainly English-speaking southern half of Wales.
Carmarthen was lost again in 1979, and the campaign to win self-government for Wales, towards which Plaid Cymru had been somewhat ambivalent, was temporarily abandoned after the referendum on devolution was lost. After this setback, Plaid Cymru continued to make inroads. Two younger men, Dafydd Wigley and Dafydd Elis Thomas, won Parliamentary seats, inheriting the mantle of leadership in turn, and the party gained ground until the successful devolution referendum of 1997, following which the Welsh Assembly was set up. Plaid Cymru became the main opposition to the ruling Labour group in this new body.
In the elections of May 2003 Plaid suffered a disaster. Of their 17 AMs only 12 were re-elected. Labour won back their traditional strongholds of the Rhondda and Islwyn, but also made shock gains in Llanelli and Conwy, seats that Plaid were expected to hold on to. Ieuan Wyn Jones resigned as party leader on the 8th of May.
On 15 September 2003 the well-known folk-singer and long-time activist Dafydd Iwan was elected Plaid Cymru's new President, while Ieuan Wyn Jones was re-elected party leader in the Welsh Assembly (both roles being held by Ieuan Wyn Jones before 8 May 2003).
Some party members have been critical of Dafydd Wigley's tactics of distancing Plaid from its traditional defence of the Welsh language, and blame Plaid's decline on this, particularly in the Party's heartland of North West Wales. In the 2001 General Election, Plaid lost Wyn Jones' old seat of Ynys Mon. Wigley's old seat of Caernarfon is very vulnerable to the Labour Party, Carmarthen is even more vulnerable, and the resurgence of the Liberal Democrats puts Ceredigion at risk.
Since the 2003 Assembly elections Plaid have reassessed their policy in relation to Welsh independence and now expressly support this as their constiutional aim for Wales.
Plaid retain close links with the Scottish National Party, with both parties' MPs co-operating closely with one another.