SAN LORENZO DE EL ESCORIAL, Spain—When María Oliva Ibáñez attended dictator Francisco Franco’s interment in 1975, she thought she was witnessing the end of an era that would remain buried for good, like him.

But now, more than four decades later, the 85-year-old is angry that Spain’s center-left government is forcing her to say a second goodbye to Franco. Prime Minister Pedro Sánchez’s plan to exhume Franco before the year’s end from a shrine-like monument, and rebury him somewhere less grandiose, is politically opportunistic...