This would have major butterflies for the Spanish transition to democracy. If Portugal goes red after a democratic revolution then the Spanish military and conservative Francoist leadership would think that it would happen in Spain too.
I can see a crack down on the protests happening and the Prime Minister at the time, Carlos Arias Navarro, siding with the military and their representatives in parliament (El Búnker) which would strangle the OTL transition before it really started.
Although the pro-democracy King would have had massive sway in the army their fear of a repeat of the Portuguese democratic revolution turning communist would probably make them ignore the King.
Therefore you could have a Communist Portugal being invaded by Spain under military rule, supported by NATO due to reasons Tocomocho said above.
How long the military rule in Spain could last, I don't know. The Spaniards were yearning for democracy and to become part of Western Europe. However, if their chance after the death of Franco is stopped, I don't know when it would come again.