When President Trump imposed eye-watering tariffs on Brazil to help Jair Bolsonaro, a political ally, Latin America’s largest nation kept a cool head and bet on old-school diplomacy.

But on Monday night, that approach was imperiled when the Brazilian Supreme Court justice overseeing Mr. Bolsonaro’s case, Alexandre de Moraes, ordered his house arrest and barred him from using a cellphone.

The United States quickly condemned Justice Moraes’s decision. The U.S. State Department’s Bureau of Western Hemisphere Affairs posted a scathing message on X: “Let Bolsonaro speak!” It also accused Justice Moraes of using “Brazil’s institutions to silence opposition and threaten democracy.”

The escalation in tensions over Mr. Bolsonaro’s case risks deepening the biggest diplomatic crisis to emerge in decades between the Western Hemisphere’s two most populous nations. It also threatens to derail trade negotiations just before 50 percent tariffs are set to go into effect this week, which would make many Brazilian products more expensive for American consumers.

President Luiz Inácio Lula da Silva of Brazil had made clear that Mr. Trump could not meddle in the case of Mr. Bolsonaro, the former Brazilian president who is accused of trying to stage a coup. Mr. Lula said he just wanted to talk trade.

“Brazil is eager to negotiate on trade,” said Thomas Traumann, a political analyst and former press secretary for Dilma Rousseff, a leftist former Brazilian president. “And, obviously, any decision from Alexandre de Moraes just makes it so much more difficult.”