Since March, the son of the former Brazilian president Jair Bolsonaro has been roaming the halls of the White House.

Eduardo Bolsonaro, 41, is a sitting Brazilian congressman, but his latest job has been to convince U.S. officials that a dangerous Brazilian Supreme Court justice wants to throw him and his father into prison, simply for fighting against what they claim was a stolen election.

And in multiple visits to Washington over the past several months, he has found a sympathetic audience.

“This is nothing more, or less, than an attack on a Political Opponent — Something I know much about!” President Trump posted online Monday. “It happened to me, times 10.”

Eduardo Bolsonaro said he had been pushing senior White House officials to place sanctions on the Brazilian judge overseeing his father’s prosecution.

Then, on Wednesday, Mr. Trump opted for something far more damaging: a 50 percent tariff on all Brazilian imports starting Aug. 1, retaliation for what he called a “witch hunt” against Jair Bolsonaro.