The Catalan government is expected to hold snap elections if it loses the vote, which could imperil separatists parties' majority in the regional parliament.
As a controversial referendum on the independence of Catalonia draws near, the Spanish government has expanded efforts to shut it down, even blocking access to some websites.
Despite legal action orchestrated by Madrid, Spain's capital and seat of power, separatist officials in Catalonia, including regional president Carles Puigdemont, have pushed ahead with plans to hold the referendum on October 1. "All the referendum's logistics have been dismantled", the Spanish government's representative in Catalonia, Enric Millo, told reporters on Tuesday. "You are talking about staying with a truly great, handsome and very historic country", Trump had said.
"We call on you and your government to engage with the Catalan government as partners in facilitating a democratic and just resolution to this situation".
Police also went to the office of Oliana Mayor Miguel Sala, one of 700 mayors under investigation for saying they would cooperate with the referendum.
Spanish security forces were deployed to neutralise polling stations across the region seeking independence from the country.
Many, including the central government, also say that a wide-ranging constitutional reform is also needed that would include specific articles to protect the Catalan language and culture and would be approved by a national referendum.
Catalonia and the neighboring Basque region have long expressed the desire to become independent of Spain, with the tension occasionally spilling over into violence.
President Trump's decision to chime in reflects the largely anti-secessionist views across the West right now, with many nations fearing they'd face similar votes if Catalonia succeeded.
Nonetheless, the percentage of Catalans wanting independence has not risen in recent years.
Whether that strategy will be seen as a critical step on the path toward independence, or a step back, only time will tell.