President Trump said on Wednesday that he planned to impose a 50 percent tariff on all Brazilian imports, partly in retaliation for what he sees as a “witch hunt” against his political ally, former President Jair Bolsonaro, who is facing trial for attempting a coup.

In a letter to President Luiz Inácio Lula da Silva of Brazil, Mr. Trump wrote that the new tariffs would take effect on Aug. 1. “The way that Brazil has treated former President Bolsonaro, a Highly Respected Leader throughout the World during his Term, including by the United States, is an international disgrace.”

Mr. Trump’s effort to use tariffs to intervene in a criminal trial in a foreign nation is an extraordinary example of how he views levies as a one-size-fits-all cudgel.

In his letter, Mr. Trump said the 50 percent tariff was needed to “have the Level Playing Field we must have with your Country” and “to rectify the grave injustices of the current regime.”

The United States had a $650 million trade surplus with Brazil in the first three months of the year, on roughly $20 billion in trade between the two nations, according to the American Chamber of Commerce for Brazil.

On Monday, Mr. Trump publicly criticized the criminal case against Mr. Bolsonaro, which focuses on the former president’s attempts to hold on to power after losing Brazil’s 2022 election.