MADRID — One of the worst mistakes by Prime Minister Mariano Rajoy's Cabinet has been to underestimate how far Catalan separatist leaders are prepared to go for the sake of independence, warns Albert Rivera, the Catalan leader of the liberal Ciudadanos party.

“I believe Rajoy didn’t manage to capture how grave [the situation] was and how much populism was growing” in Catalonia, Rivera — whose party is against secession — told POLITICO in the run-up to the independence referendum set to take place Sunday, which has been deemed illegal by Madrid.

“We need to acknowledge that the separatists … have been doing what they said they would do,” Rivera said. “The [central] government said they wouldn’t call the referendum, and they called it.”

The Ciudadanos leader, who calls the separatist leaders “pyromaniacs” and “putschists,” warned that Spain is undergoing a crisis that threatens the rule of law.

“Anything can happen,” Rivera said.

The regional government led by Carles Puigdemont has called on Catalans to take to the streets and cast their ballots on Sunday to decide whether they want to break away from Spain. In response, the Spanish courts have ordered national and regional police to seal off polling stations to prevent the vote. The central government has deployed thousands of police in the region in anticipation of potential unrest.

“If you decide to jump off the cliff, of course, you will feel the blow" — Albert Rivera

Rivera argued that the actions taken by the judiciary to prevent the vote — including criminal charges against dozens of Catalan officials — are reasonable given the circumstances.

“I don’t think anyone in the Catalan government can be surprised when a president that says that he doesn’t respect judicial decisions is criminally prosecuted,” he said.

“If you decide to jump off the cliff, of course, you will feel the blow," Rivera said. "And in this case, the blow is the penal code, the justice [system], the rule of law.”

Puigdemont and his allies disagree, denouncing a "totalitarian" crackdown by the Spanish state.

Rivera’s Ciudadanos, the Socialists and Rajoy’s Popular Party — which make up three-quarters of the Spanish Congress — disagree on who is to blame for the current crisis and how to tackle the Catalan question. But they all back the judiciary's efforts to abort what Rivera described as “a process of radicalization outside the law.”

Spain’s 1978 constitution — which was ratified by an overwhelming majority of Catalans — establishes the principle of the united sovereignty of the Spanish people. A large majority in the national parliament opposes constitutional change to endorse the right of self-determination for Catalonia — or any other region.

'Good and bad Catalans'

“I frankly don’t think it is a good idea to decide every four years if we’re going to dissolve our country,” Rivera said.

Spain is trapped in a difficult dilemma, said Pablo Simón, a politics professor at Carlos III University in Madrid.

Madrid is seeking to reestablish “the rule of law in Catalonia,” where the government has decided “to selectively break the Spanish law.” But the stricter it applies the rule of law, the more it risks inflaming existing tensions — and the more difficult a political solution is likely to become.

These days, few politicians dare claim to know what will happen in Spain in the days and weeks ahead.

“The million dollar question right now,” Rivera said, is whether Puigdemont and his allies “will go ahead with their promise” of a unilateral declaration of independence.

“I hope they don’t, because, if they do, this will become an even bigger disaster, he said. It would force the central government to take control over the Catalan executive, and "Puigdemont would have de facto destroyed Catalan autonomy," he said.

Rivera, who founded Ciudadanos as a regional party in Catalonia before it went national in 2015, said he spent years in the region's chamber fighting nationalist lawmakers who accused Spain of stealing Catalan money and resources.

When the economic and financial crisis hit the country in 2007 — pushing the unemployment rate to a peak of 27 percent — Catalan nationalists saw an opportunity “to break up Spain,” Rivera argued.

In the past decade, support for independence in Catalonia grew from around 15 percent to around 41 percent today, according to the Catalan government-funded Center d’Estudis d’Opinió.

As a Catalan advocating for Spanish unity, Rivera has become the target of attacks from independence supporters. In 2007, two members of the youth division of the Catalan Republic Left — one of the ruling parties in the current regional government — sent Rivera a menacing letter at his home accompanied by a photograph of him with a real bullet pushed through the paper. They were later thrown out of the party.

This month, a takeaway food shop in Granollers owned by Rivera's mother and aunt was plastered with graffiti that read: “Ciudadanos, this is neither your land nor your struggle.”

“Nationalism is generating a divide between good Catalans, who are those who support secession; and bad ones or enemies … who are those of us who want to remain Spanish and European,” said Rivera.