Buena pregunta, estimado Pikers....
Well, regarding the moderation of the PCE, if you want a more extreme party you probably need an earlier POD. On one hand, we have the party embrasing a proto-eurocommunism in the late 60's (comdemning, besides the PCI, the soviet intervention in the Praga Spring), and as "founder" member of Eurocommunism besides the PCI and the PCF in 1977. To change this, you would need another person different to Santiago Carrillo leading the party (Ibarruri?), so the POD should be, at least, before 1960.
On the other hand, we have the nature of the communist militancy inside Spain in the 60's and 70's. Since the PCE and satellite organizations were the ones with the biggest, best funded and most active antifrancoist structure in Spain, it attracted many people who, not being necessarily communist, found in the sein of the party a place to fight against the dictatorship. That's probably the reason that explains the divergence amongst the importance of their role during the dictatorship and their electoral results once democracy was restored. Perhaps, if other opposition forces had managed (or cared, depending on the cases) to have a more active role in the clandestine opposition, the PCE would have been a more radical force, but in that case we have a very different scenario with important butterflies in all fronts once Franco dies, if he manages to die in power.
Finally, inside the party there was also a clear conscience of the situation, and the fact that some extremist forces were trying to create a "logic of confrontation" to derail the transition proccess. So, for example, after the 1977
Massacre of Atocha (it was 35 years ago yesterday, by the way) the communist reaction, instead of rioting in the streets or claiming vengeance, was a solemn, silent massive march in Madrid during the funerals, earning the solidarity of the public and showing their force at the same time, though this point is probably a consequence of the former two.
Meawhile, on the other side of the galaxy, it has to be noted that the legalization of the Communist Party certainly was a risky, somewaht personal bet made by Suárez (whch, besides other things, had ulterior political consequences for him, but that's perhaps another story). So, another person could have behaved on a different way or Suárez could have felt the pressure from the bunker in the last moment (perhaps butterflying the Atocha Massacre?) though the latter is more unlikely, Suárez was many things, but not a coward.
So, I'm assuming the latter option (the government doesn't legalice the party, instead of the party being more radical), since it seems easier to reach. The "strategy of confrontation" would have it more easier to make things difficult in Spain, and the moderation inside many sectors of the PCE would have been difficult to be kept, specially if there is not hope of legalization. The situations seen during the 1976 strikes probably would return reinforced. The problem is not only a clandestine PCE, is that the main union, Comisiones Obreras, had close links with the party, so their ability to create trouble was big. On the other had, keeping the PCE out of the law would make difficult the homologation of the spanish democracy with those inf the rest of Western Europe, which was the main objetive of the reformists so, from that POW, the transition would be over and spain would be more isolated.
Now, how much time are you thinking the state is going to resist the pressure from the left? I foresee two possibilities:
The Socialist Party wins the elections in 1979, since the UCD is not seen by many voters as a credible reformist force and a disaster for the economy (see below). The new governement legalices the PCE, if the military doesn't enter in panick mode and try something, whose result I couldn't say.
The center-right stays in power like in OTL, but the conflictivity skyrockets in relation to OTL, most probably the
Moncloa Pacts are not possible, increasing the labour tensions and difficulting a coherent economical policy, which is not good in the middle of a deep economical crisis. With the communists radicalizing and the rest of the left being skeptical about the reformist will of the government, perhaps we have a more active and bigger
GRAPO or similar groups. At some point, the government has to opt for a political solution or the situation could degenerate very dangerously, not necessaryly leading to an implossion, but probably slowing the proccess of democratization some years.
If I can I'll give you some sources latter.
Cheers.
