"I really think the people of Catalonia would stay with Spain," Trump said as he offered support to visiting Spanish Prime Minister Mariano Rajoy at the White House Tuesday.
"I think it would be foolish not to. You're talking about staying with a truly great, beautiful and very historic country."
Rajoy's government says the referendum is illegal under the Spanish constitution.
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Police have seized nearly 10 million ballots for the vote and have closed down 59 websites that provide information about the referendum.
Trump suggested accurate polls were difficult to come by. "I think the people of Catalonia have been talking about this for a long time," he said. "I bet you, if you had accurate numbers and accurate polling, you would find they love their country. They love Spain."
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".
Rajoy urged Catalan officials to return to "common sense" while announcing that the crisis would delay the national budget.
Rajoy, who has also been criticized for his approach to the crisis, ruled out fresh elections.
"I'm not thinking about calling early elections as a result of what we were seeing.".