It also once had a stranglehold on dozens of working-class cities surrounding Paris, an area known as the Red Belt, where the architecture had a tinge of socialism, the streets were named “Lénine,” and the party organ, L’Humanité, was delivered to residents’ doors.

Perhaps no community has been more committed to the cause than Ivry-sur-Seine, an eastern suburb of Paris that has been governed by the Communists since 1925. It is part of Val-de-Marne, the last of France’s 101 departments to still be governed by Communists.

Ivry was where the party built an ambitious housing project in 1961 that embodied its ideals for its working-class supporters: a huge 14-story, T-shaped red brick building with 382 modern apartments and subsidized rents.

As a symbol of the party’s seemingly boundless future, the building was named Cité Gagarine, after the Soviet cosmonaut Yuri Gagarin, the first man in space. The building’s renown was cemented after Mr. Gagarin paid a visit in 1963, and residents threw rose petals at him.