This is fairly hard to evaluate. In the
1975 election they only got 12.5% of the votes (PCP) plus 4.5% for the MDP. In
1979, tough, the two parties combined (as the United People's Alliance, APU) got up to 19%. This may not seem much, but Lenin also only garnered 23% in 1917 Russia...
Anyway, it is important to note that this was all after the 25 November coup and the events of the Hot Summer had greatly tarnished the communists' reputation. Before the election actually happaned, I think everyone was expecting them to fair much better. They had massive popular support in south of the country, especially in Alentejo and in the industrial areas south of river Tagus. It was really just in the north and in some parts of Madeira and Azores that they were really unpopular.
In my opinion there was more than enough support coming from the militairy and certain sectors of the population to sustain a communist regime. All it'd take would be a few tweaks here and there and screwing the moderate left.