So Portugal was ruled by a dictatorship throughout most of the 20th century.
What if the junta had resisted the carnation revolution and fought to the bitter end.
The dictatorship had two phases, a military dictatorship and a civilian single-party dictatorship. The military dictatorship lasted from 1926 to 1932, and since 1926 until 1974, it was a civilian single-party dictatorship.
There was no support left for it by the time it fell. All the excuses in which it was based were no longer believed, there was no more danger of the 1st republic returning (nor did the opposition wanted it). Everyone was fed up of the economic straight jacket of the "industrial conditioning" defended by the dictatorship (that limited the competition among key sectors of the economy), of the lack of personal freedoms, or the never ending colonial war.
The Estado Novo did not have majority support since at least the end of world war 2.
Could the soviets support a guerilla movement?
Other consequences.
There was neither the support, nor it is easy to issue a guerilla against a modern army in a relatively small country.
Additionally, a Soviet-supported movement, which would imply a communist-leaning movement, which would be anathema for the vast majority of the country, and the vast majority of the population, and the part of the country where they could find support would be, IMHO, the less suited for a guerilla movement. Such movement wouldn't be supported by the vast majority of the opposition (ranging from centre-left to monarchists), who were deeply distrustful of the communists.