directly from the article:
"But one of the most lasting abuses of the era was borne by children. In the late 1930s and 1940s, Antonio Vallejo-Nájera, a leading psychiatrist in the regime who was trained in Nazi Germany, promoted the idea of a Marxist “red gene” carried by the children of Franco’s left-wing opponents. The gene, he said, might be suppressed by removing children from their mothers and placing them with conservative families. Franco’s men soon began the abductions on a large scale. They targeted children orphaned by Franco’s firing squads and took newborns belonging to women who had given birth in jail as political prisoners. All were sent to be raised by regime loyalists. The era of the “stolen babies” had begun."
the transfer of the babies, predominantly from low class families to high ranking Catholics, allies of Franco's regime of course makes the ideological connection clear.
Antonio Vallejo-Nájera was explicit about the political/eugenic character of these transfers:
"Vallejo’s conclusions were that the only way to prevent the racial dissolution of the Spanish was to take away the red children from their mothers in places "away from democratic environments and where the exaltation of bio-psychic racial qualities is encouraged". By 1943, 12,043 children had been taken from their mothers and handed over to orphanages or Francoist families"
https://en.wikipedia.org/wiki/Antonio_Vallejo-N%C3%A1jera