Abraham Lincoln suggested the United States was “the last, best hope of Earth.” Ronald Reagan celebrated it as a “shining city on a hill.” George W. Bush argued that the nation was “the brightest beacon for freedom and opportunity in the world.”

But to President Trump, America is the all-powerful player in a series of high-stakes transactions.

“You don’t have the cards right now,” he lectured President Volodymyr Zelensky of Ukraine in their extraordinary Oval Office showdown.

Mr. Trump is radically different from his Republican predecessors in countless ways. But rarely is the contrast starker than in his approach to American leadership in the world.

While those Republicans sometimes spoke of global alliances in terms of good and evil, Mr. Trump joined with America’s adversaries to oppose a United Nations resolution condemning Russia’s invasion of Ukraine, and falsely suggested that Ukraine had started the war. While they championed free trade, he has started trade wars. And while they argued that American assistance abroad could fend off problems at home, he has moved to gut foreign aid.

Now, Mr. Trump’s nakedly transactional style is forcing Americans to reimagine how they see their country’s place in the world, according to interviews with roughly two dozen voters, foreign policy experts and current and former elected officials from across the country.

“I fear we are risking what made us great over the last 80 years, since the close of the Second World War,” said R. Nicholas Burns, a former ambassador to NATO and China. “Every president until now adhered to democratic principles and values, including the idea that America should be the standard-bearer for democracy around the world, to uphold the rule of law and defend the sovereignty and territorial integrity of each country.”