With five days to go until the October 1 vote, the clash between Catalonia's pro-separatist government and Madrid was increasingly being played out in the arena of logistics and international opinion.
During a joint news conference with Spanish Prime Minister Mariano Rajoy in Washington, Trump said it would be "foolish" for Catalonia not to stay in Spain.
"You're talking about staying with a truly great, beautiful and very historic country," he added as Rajoy stood at his side.
Rajoy urged Catalan officials to return to "common sense" even as Madrid stepped up its effort to stop the vote from going ahead.
The chief prosecutor in Catalonia ordered police to seal off buildings that will house polling stations before the day of the referendum and deploy officers on the day of the vote to prevent ballots from being ast.
The move comes a day after he ordered regional police to identify those in charge of polling stations on Sunday, when the referendum is to be held.
"The order has been conveyed and it will be executed will all normality," a spokesman for Catalonia's regional police force, the Mossos d'Esquadra, told AFP.
To ensure they will cooperate, Spain's interior minister this weekend put Catalonia's regional police force under its supervision.
'Losing the battle'
By focusing on polling stations, prosecutors appear to have put in place a plan that targets all the logistics needed to stage the referendum, which has been deemed illegal by Madrid.
Prosecutors have also threatened Catalan mayors who provide locations for the vote with criminal charges, as well as directors of schools and universities.
The election commission set up by Catalan separatists to oversee the vote has resigned after Spain's Constitutional Court threatened to impose daily fines of 12,000 euros ($14,100).
Police have seized nearly 10 million ballots for the vote and have closed down 59 websites that provide information about the referendum.
Another 85 sites are in the process of being closed, judicial sources said.
Faced with these actions, the separatist leaders of this wealthy northeastern region of Spain, home to around 7.5 million people, have accused Madrid of "repression".
The website of the foundation of former Spanish dictator Francisco Franco "remains operational" but not the referendum websites, said Catalan government spokesman Jordi Turull.
The central government argues that it is simply applying the constitution, which does not allow this type of referendum, just as in neighbouring France and Italy.
Spain's democratic constitution of 1978, which was approved by more than 90 percent of Catalan voters, gave wide autonomy to the regions but affirmed "the indissoluble unity of the Spanish nation".
'Vote en masse'
Rajoy has pulled out of an informal summit of European Union leaders in Estonia on Friday so as to be able to attend the last meeting of his cabinet before the referendum.
He also announced that the crisis would delay the national budget but ruled out fresh elections.
"I'm not thinking about calling early elections as a result of what we were seeing," he said in Washington.
As the date of the referendum nears, tensions are rising.
In Huelva in the southwestern Spain, Guardia Civil officers dispatched to Catalonia for the referendum were cheered on by hundreds of locals with cries of "Go for them!" and draped them with Spanish flags.
In Barcelona, residents have been giving out red carnations to regional police.
Spain has deployed two-thirds of its riot police to Catalonia, some 2,000 officers, for the referendum, according to daily newspaper El Pais.
Despite the crackdown, Catalonia will vote on Sunday in an independence referendum, regional foreign minister Raul Romeva told AFP.
"People will go out and vote en masse, peacefully on Sunday," he said. "I don't have any doubt."
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