I think Stalin sabotaged the Republicans cause since he was afraid of disturbing UK, they sure did not want a commy state as neighbour. Imagine UK joining up with Germany against communism in Europe.
I think that over-zealous Comintern agents and an NKVD infiltrated PCE (the communist part of Spain) are what really caused the most damage to the Spanish Republic. The wasting of men and material on useless “prestige” offensives in places like Brunete and Teruel, the lack of initiative shown the Soviet advisors and their communist promoted officers, and the persecution of Republic supporting groups like the CNT, UGT (anarchists) and the POUMS (Spain’s Marxist/trotskiest party) did the most damage.
Also, the other single biggest enemy of the Republic and the country that did the most to help the Nationalists and hurt the Republic was the United Kingdom. Their policy of “non-intervention” and the stupid committee they set up that did absolutely nothing. Even when given proof that both Germany AND Italy were blatantly ignoring their roles in this non-intervention committee, and actively helping Franco with troops and equipment, the UK did nothing. Also Britain keep France from helping the Republic by suggesting that they would not back France against the Germans if they did. It was part of the UK’s stupid policy of appeasement that did the most damage to the Republic and thrust them into the arms of the Soviets, and all the damage that caused.