Spanish police have confiscated millions of ballots in recent days as part of a crackdown to stop the October 1 vote, which has been suspended by Spain's Constitutional Court.
Despite long-standing assurances from the State Department that the USA would take no stance on the upcoming Catalonia referendum, President Trump couldn't help himself while hosting Spain's Prime Minister, loudly endorsing Spanish unity.
On Tuesday, the Spanish government's representative in Catalonia's capital Barcelona, Enric Millo, warned that Catalan president Carles Puigdemont could be removed from office if he continued to push ahead with the referendum, planned for this weekend.
The referendum in the northeast region of Spain is scheduled to be held on Sunday despite the Spanish Constitutional Court declaring it illegal.
"I'm just for a united Spain", said Mr Trump, who cast doubt on polling data predicting a "yes" vote for independence will win.
Spanish Deputy Prime Minister Soraya Saenz de Santamaria again insisted on Wednesday that the Catalan independence referendum will not take place.
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Spain's former ambassador to the United States was unequivocal in his response to statements from the White House suggesting the US would recognize a Catalonia independent from Spain. What is clear is that even if the vote was taken and did result in a call for independence, Catalonia would not be recognized by any other state.
Spanish authorities want the names of officials running polling stations at schools and civic buildings, with a view to prosecuting them.
The central government has described the move as illegal.
The independence movement is widespread enough that it is unlikely to dissipate if the regional government fails to convert a "yes" vote into a split from Spain.
Many had not yet received information about where or when they would be working after the state-run postal service was told to stop all mail related to the vote, Marta Rovira said in a radio interview. But a large majority of people are in favor of being given the right to vote on the matter, making Spain's heavy-handed approach all the more politically risky.
Although Catalonia enjoys a certain measure of autonomy, separtists have long campaigned for independence for a wealthy region with its own language and cultural traditions.