Prior joined the Cornish Nationalist Party on its formation in 1976, and was adopted as its prospective candidate for the St Ives constituency. This candidacy did not happen, as he stood down in favour of the Mebyon Kernow (MK) candidate for the constituency, and subsequently he defected to MK. He was the editor of Mebyon Kernow's journal, Cornish Nation, from 1978 to 1979, jointly with Julyan Holmes. In 1979, he stood for the St Ives South seat on Cornwall County Council; although he was not elected, he took one of the party's best vote shares.
In 1981, Prior was elected as Treasurer of Mebyon Kernow, and at the 1983 UK general election, he was the party's candidate for the St Ives seat, taking fourth place with 1.2% of the vote. He was elected as Chairman of the party in 1985, serving for one year.
- Bernard Deacon and Garry Tregidga, Mebyon Kernow and Cornish Nationalism, p.130
- James Whetter, Cornish essays, 1971-76, p.138
- Bernard Deacon and Garry Tregidga, Mebyon Kernow and Cornish Nationalism, p.58
- Carn, vols. 25-32 (1979), p.41
- Carn, vol. 36, p.18
- F. W. S. Craig, Britain Votes 3, p.55
|Party political offices|
|Leader of Mebyon Kernow