Alexandre de Moraes, a Brazilian Supreme Court justice, is a political Rorschach test of sorts.

Over the past several years, he has undertaken an aggressive campaign to rid the Brazilian internet of what he says are threats against Brazil’s democratic institutions, ordering the removal of hundreds of social media accounts in the process, nearly all of them right-wing.

As a result, the left in Brazil considers him a savior of the nation’s democracy, helping to protect it against an attempted coup in 2022.

To the right, he is a dangerous government censor who has abused his power to silence conservative voices online.

Elon Musk has challenged him, President Trump’s media company has sued him, and the former Brazilian president, Jair Bolsonaro, is accused of plotting to assassinate him.

Now, for the first time, he appears to be in the cross hairs of the U.S. government — a development that could cause a diplomatic rift between the Western Hemisphere’s two largest nations.

On Wednesday, Secretary of State Marco Rubio announced that the United States would restrict visas from foreign officials who are “responsible for censorship of protected expression in the United States.” In an online post, Mr. Rubio noted Latin America as an example of a region where he believed such censorship has been a problem.

The State Department would not say whether Justice Moraes is a target of the action. But the language of the State Department policy skews closely to complaints about him, including that visas would be restricted from officials who demand that U.S. tech companies take down content or threaten arrest warrants against U.S. residents for things they said online.

Mr. Rubio also told Congress last week that U.S. officials were studying whether to place sanctions against Justice Moraes and that there “is a great possibility that will happen.”

Separately, the Justice Department sent a letter to Justice Moraes this month that scolded him for ordering Rumble, a U.S.-based social network popular with conservatives, to block the accounts of a specific user, according to a copy of the letter viewed by The New York Times and that has not been previously reported.

The Justice Department told Justice Moraes that he could enforce laws in Brazil but that he could not order companies to take specific action in the United States.

A spokeswoman for Justice Moraes declined to comment.

Any action by the Trump administration against Justice Moraes could destabilize the bilateral relationship between the two countries, which has already been tested by Mr. Trump’s election.

Justice Moraes is the lead judge in the federal criminal trial that will decide whether Mr. Bolsonaro attempted to stage a coup and hold on to power following his election loss in 2022. Mr. Bolsonaro has been one of Mr. Trump’s closest allies among current or former world leaders, and he has publicly called on Mr. Trump to help rescue him from what he says is political persecution.

In February, a day after Mr. Bolsonaro was indicted in the coup case, The Trump Media & Technology Group — which is majority-owned by Mr. Trump — and Rumble sued Justice Moraes, accusing him of ordering Rumble to block the accounts of a Brazilian living in the United States who is seeking political asylum.

Mr. Rubio told Congress last week that U.S. officials were studying using the Global Magnitsky Act to place sanctions against Justice Moraes. That would be an extraordinary measure, given that the Magnitsky Act is designed to punish foreigners accused of serious human-rights violations or corruption, and that Justice Moraes is a sitting Supreme Court justice in good standing with the Brazilian judiciary and government.

After Mr. Rubio’s testimony, Brazilian diplomats immediately began engaging with their U.S. counterparts, eventually resulting in high-level talks involving Brazil’s foreign minister, Mauro Vieira, according to a Brazilian government official who spoke on the condition of anonymity to discuss private talks.

The Brazilians stressed that Justice Moraes’s orders to remove online accounts had been taken in the context of a serious threat to Brazil’s democracy, the official said, given that the authorities have since uncovered a plot that called for a military intervention and that Mr. Bolsonaro’s supporters raided the nation’s halls of power a week after President Luiz Inácio Lula da Silva took office.

The announcement of a policy to impose visa restrictions, instead of sanctions under the Magnitsky Act, was viewed as far more palatable by the Brazilians, the official said.

It was unclear if the United States has taken or plans to take any action against Justice Moraes.

The State Department visa policy focuses on foreign officials that try to force U.S. companies to take action in the United States. Martin De Luca, Rumble’s lawyer, said that Justice Moraes ordered Rumble to block a user’s accounts in the United States. A U.S. federal judge said Justice Moraes had not sought to force Rumble to take action in the United States and that his orders are largely only applicable in Brazil.