Question: Was there any time in modern Portuguese history when this political division could have led to the actual partition of Portugal? There does seem to have been a brief period after the Carnation Revolution when it was talked about:
"The country moved towards an open geographical partition between the two sides. With order apparently unenforceable in the streets of Lisbon, with the left on the ascendant in the south and in the capital, the north seemed to be the last bastion of democracy. In Oporto that same night, eleven people were injured as anti-Communist crowds sacked Communist buildings. In Lisbon the Communists called a mass demonstration on 15 November to celebrate their victory. Most of the moderate military men and the democratic political leaders absented themselves discreetly from the capital for the weekend. Leaders of all three democratic parties met in secret session in Oporto. And on 17 November, 253 non-Communist members of the constituent assembly met in Oporto to discuss transferring their sessions there permanently. As one member put it: 'The mob rules Lisbon. Nobody governs it.'" https://books.google.com/books?id=aa6uCwAAQBAJ&pg=PA88
The Communists and their allies, as we know, were defeated--but what if they hadn't been? Could the north have successfully separated (presumably with US support, covert or otherwise)? I suppose that if that happened the US would argue that north Portugal was the legitimate Portuguese state and as such had the rights to the Azores, strategically important to the US...