Spain is determined to prevent a Catalonian referendum on independence, even though its tough attitude toward Catalan leaders has probably only increased enthusiasm for a measure with questionable support.

A new round in a long game of chicken began earlier this month, after Catalonia’s president, Carles Puigdemont, announced an independence referendum on Oct. 1. Spain’s culture, sport and education minister, Íñigo Méndez de Vigo, vowed: “What I can say is what will not happen on Oct. 1 — an illegal referendum that goes against the Constitution.”

After Catalonia’s government staged a nonbinding independence vote in 2014, Spain charged the autonomous region’s leader at the time, Artur Mas, with the crimes of disobedience and breach of trust. In March, a court fined Mr. Mas the equivalent of $39,000 and banned him from holding public office for two years. The trial only succeeded in galvanizing Catalan separatists, as hundreds of thousands took to the streets in September to demand that their politicians push ahead on independence.

But while independence was approved in the 2014 ballot measure, less than half the electorate took part, and many Catalans see clear advantages in remaining a part of Spain, such as membership in the European Union.