ARENYS DE MUNT, Spain — As Spain’s government tries almost everything it can to stop an independence referendum Sunday in the restive northeastern region of Catalonia, the standoff is escalating into a constitutional crisis emblematic of the larger forces tearing at European unity.
With the support of the Spanish judiciary, Madrid has shut down websites and advertising campaigns that have promoted the vote. It has raided the offices of companies that would print the paper ballots. It has sent in thousands of police officers from outside the region, threatening to block polling stations.
Last week, a dozen regional government officials were detained. Spain’s attorney general has warned that scores more could be arrested and prosecuted, including even the leader of Catalonia, Carles Puigdemont.
“We are witnessing the worst democratic regression since the death of Franco,” Mr. Puigdemont said in an interview, referring to Gen. Francisco Franco, the dictator whose death in 1975 opened the way for Spanish democracy. “What is happening in Catalonia is very serious.”
Indeed, Catalonia’s standoff has steadily, if quietly, ratcheted up this year as world attention largely focused elsewhere, with pivotal elections taking place in critical European Union states, most recently last Sunday in Germany.
Those elections presented big tests of the bloc’s cohesion. The dispute over Catalonia instead presents a test of the cohesion of a member state itself. And it points to ominous storm clouds in other independent-minded regions, from Scotland to northern Italy.
Catalonia, which has its own language and culture, has long been an uneasy part of Spain, and aspirations for independence have surged and ebbed for generations. But Catalonia is also Spain’s economic engine, and Madrid is determined to hold onto it. The tug of war is now entering perhaps its most intense and unpredictable phase since the approach of the Spanish Civil War last century.
In 2014, the last time Catalonia held an independence vote, it, too, was declared illegal by Spain’s constitutional court. But Catalan officials described that vote as a straw ballot, and the government in Madrid and the police did not prevent it.
This time, sensing the growing seriousness of the Catalan referendum, which the regional government says will now be binding, Madrid is taking a far more aggressive tack.
The approach has left many Catalans bridling under what they say is a heavy hand by the government of Prime Minister Mariano Rajoy. The steps to intimidate people who back a vote not only may backfire, they argue, but also threaten to transform the conflict into a broad campaign of civil disobedience that could spiral out of control.
Tensions on the streets have mounted, along with political recriminations. While the separatists charge that Mr. Rajoy is taking Spain back to the dark days of Franco, Madrid warns that the separatists have shifted from violating Spanish law to encouraging civil strife.
In the interview, Mr. Puigdemont said the conflict would not turn violent, but he warned that Madrid would have to assume its share of the blame if things got further out of hand. “If you stop somebody from unfolding a banner that asks for more democracy, the problem is with the person who forces its withdrawal,” he said.