During the 1971 No-Vote campaign the PNM won all 36 seats and received about 22% of the eligible vote. That marks their core support – about 22% of the electorate. According to preliminary figures, the PNM received 236,420 votes out of an electorate of 990,352. In other words, 23.9% of the electorate.
Update: Newer figures show that the PNM received 46% of the votes (299,813 votes) cast and the support of 30% of the electorate. Turnout was a fairly normal 66%, not an unusually low 56%.