"Mexico's going to pay for the wall," he said. "I love the Mexican people; I respect the Mexican leaders, but their leaders are much sharper, much smarter and more cunning than our leader."
For years, the issue has divided anti-immigrant conservative activists from business-oriented Republicans and the fast-growing Hispanic constituency, from which the GOP needs more votes to win presidential elections.
But Trump widened that divide — and began his rise to the top of Republican polls with hot rhetoric in his presidential announcement speech about who was crossing America's southern border.
"When Mexico sends its people, they're not sending their best," he declared in June. "They're sending people that have lots of problems, and they're bringing those problems with us. They're bringing drugs. They're bringing crime. They're rapists. And some, I assume, are good people."
The latest Monmouth University poll — released Monday — showed Ben Carson leading the field in Iowa by double digits, while Trump held tight in second place.
— CNBC's John Harwood and Reem Nasr contributed to this report.
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