In the eyes of many of his fellow Greenlanders, Jorgen Boassen is a traitor.

A few weeks ago at a dive bar in Nuuk, Greenland’s capital, someone slugged him in the face, sending him to the hospital. But whatever the consequences of his convictions, he insists he isn’t scared.

“The United States has my back,” he said.

Mr. Boassen, 51, a former bricklayer, is a fervid supporter of President Trump. He campaigned for him in the United States and helped coordinate Donald Trump Jr.’s visit to Greenland this year. On his coffee table at home, three pristine MAGA hats occupy a place of honor.

While his championing of the American president — who has vowed to take over Greenland “one way or the other” — has made Mr. Boassen unpopular at home, it has also turned him into an unlikely political player in the Arctic, a region of growing importance in a warming world eager for its untapped resources.

As he lounged on a couch in his apartment on the edge of Nuuk, wearing a pink T-shirt emblazoned with Mr. Trump’s face, his phone buzzed with a stream of texts from journalists and filmmakers who wanted to talk and investors who hoped he was their ticket to riches in Greenland.

In the debate about the future of the world’s largest island, a semiautonomous overseas territory of Denmark, Mr. Boassen has made it his mission to bring Greenland and the United States closer together.

Still, Mr. Boassen noted he “doesn’t always agree” with the American president.

While Mr. Trump wants to claim the island for the United States, Mr. Boassen is pushing instead for a tight security alliance between an independent Greenland and Washington. That has made him one of the most visible Greenlanders agitating to break with Denmark.