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."