Daniel Silveira, a policeman turned far-right Brazilian congressman, was furious. He believed Brazil’s Supreme Court was persecuting conservatives and silencing them on social media, and he wanted to do something about it.

So he sat on his couch and began recording. “How many times have I imagined you getting beat up on the street,” he said in a 19-minute diatribe against the court’s justices, muscles bulging through his tight T-shirt. He posted the video on YouTube in February 2021, adding, “I’ll say what I want on here.”

A Brazilian Supreme Court justice immediately ordered his arrest. A year later, 10 of the court’s 11 justices convicted and sentenced him to nearly nine years in prison for threatening them.

Jair Bolsonaro, Brazil’s president at the time, pardoned Mr. Silveira, but the Supreme Court overruled him. Today, Mr. Silveira remains in prison. There is no room for appeal past the Supreme Court.

Mr. Silveira’s case is part of a creeping institutional crisis for Brazil.

For the past five years, the nation’s Supreme Court has expanded its power to carry out a sweeping campaign to protect Brazilian institutions from attacks, many of them online.

To the Brazilian left, the offensive has helped rescue Brazil’s democracy. To the right, it has made the court a threat to democracy itself.