Ada Colau, 41, was not even born when Manuela Carmena, 71, joined Spain’s underground Communist party and started her legal career by attacking labor restrictions imposed by Francisco Franco, the Spanish dictator.

But even if separated by a generation, Ms. Colau and Ms. Carmena both found themselves claiming similar left-wing victories by upstart candidates over Spain’s political establishment after Sunday’s regional and municipal elections.

Ms. Colau, a former activist who fought housing evictions, defeated Barcelona’s mayor, Xavier Trias, which she described as a “David against Goliath” victory. Ms. Carmena, a retired judge, did not win in Madrid, but her strong showing nonetheless made it very likely that she would become mayor of Spain’s capital next month, at the helm of a coalition with the Socialists.

The success of anti-establishment candidates, who ran for small local parties, in the two largest Spanish cities underlined the fragmentation of Spain’s politics, as well as the precipitous slide of the governing Popular Party of Prime Minister Mariano Rajoy, ahead of general elections this year.