The US secretary of state, Marco Rubio, has reportedly stripped eight of Brazil’s 11 supreme court judges of their US visas as the White House escalates its campaign to help the country’s former president Jair Bolsonaro avoid justice over his alleged attempt to seize power with a military coup.

Bolsonaro, a far-right populist with ties to Donald Trump’s Maga movement, is on trial for allegedly masterminding a murderous plot to cling to power after losing the 2022 election to his leftwing rival, Luiz Inácio Lula da Silva. Bolsonaro is expected to be convicted by the supreme court in the coming weeks and faces a jail sentence of up to 43 years.

As the day of judgment nears, Trump has been increasing pressure on the court and President Lula’s administration. On 9 July, the US president announced he would impose 50% tariffs on all Brazilian imports as of 1 August, partly as a result of the supposed persecution of his ally. The move triggered an outpouring of nationalist anger in the South American country, with Lula describing it as “unacceptable blackmail”.

On Friday, after federal police raided Bolsonaro’s house and fitted him with an electronic tag to stop him absconding, Rubio announced further moves in support of the defendant, who he claimed was the victim of a “political witch hunt”.

Writing on X, Rubio said he had ordered visa revocations for the judge leading the investigation into Bolsonaro, Alexandre de Moraes, as well as “his allies on the court” and their family members. Rubio did not name his other targets but the Brazilian newspaper O Globo identified them as Luís Roberto Barroso, José Antonio Dias Toffoli, Cristiano Zanin, Flávio Dino, Cármen Lúcia Antunes Rocha, Luiz Edson Fachin and Gilmar Ferreira Mendes.

Two other judges who were nominated to the court during Bolsonaro’s 2019-23 presidency, André Mendonça and Kassio Nunes Marques, reportedly avoided the sanction, as did a third judge, Luiz Fux.

Lula denounced what he called “another arbitrary and completely groundless measure from the US government”.

“Interference in another country’s justice system is unacceptable and offends the basic principles of national sovereignty and respect between nations,” the president said on Saturday, adding: “I’m certain that no kind of intimidation or threat – from whoever it may be – will compromise the most important mission of our nation’s powers and institutions, which is to act permanently to defend and safeguard the democratic rule of law.”

The Trump strategist Alex Bruesewitz welcomed Rubio’s announcement, calling Bolsonaro’s treatment “sick and wrong”.

Bolsonaro’s congressman son, Eduardo, thanked Rubio for his decision. “Thank you very much for this fight in favor of free speech, we do believe in the same values,” tweeted Eduardo, who has been living in the US since February and has reportedly been lobbying officials there over his father’s plight.

Trump’s interventions have appalled millions of Brazilians who hope to see their former leader held responsible for the alleged coup attempt, which culminated in the 8 January riots in Brasília.

Lula’s institutional relations minister, Gleisi Hoffmann, called the visa cancellations “an aggressive and petty retaliation” and “an affront to the Brazilian judiciary and national sovereignty”.

Even influential rightwing voices have criticised the US’s attempt to meddle in one of the world’s most populous democracies by imposing 50% tariffs.

On Saturday, the conservative Estado de São Paulo newspaper described Trump’s behaviour as “unacceptable external interference in Brazil’s domestic matters”. “Trump has not only attacked our national sovereignty … [but also] stained the history of diplomatic relations between the two largest democracies in the Americas,” the newspaper’s editorial board wrote.

While the Bolsonaros have hailed Trump’s actions, they also appear to have grasped how the announcement of tariffs has backfired, allowing Lula to pose as a nationalist defender of Brazilian interests and paint the Bolsonaro clan as self-serving “traitors”.

Lula, who had been facing growing public disillusionment and an uphill battle to win re-election next year, has enjoyed a bounce in the polls since Trump launched his trade war, the brunt of which will be borne by coffee producers and cattle ranchers in Bolsonaro-voting regions, such as São Paulo.

Celso Rocha de Barros, a political columnist, said he suspected the Bolsonaros had been blindsided by the scale of Trump’s attack.

“I think [Bolsonaro] wanted some kind of penalty - something he could use to say: ‘Look, Brazil’s being punished because of Bolsonaro’s persecution. But [the tariffs] went far too far … [they] screwed Bolsonaro’s base,” said Rocha de Barros, pointing to their potential impact on agribusiness.

On Friday night, Bolsonaro’s senator son, Flávio, post on X, calling on Trump to suspend the tariffs and replace them with individual sanctions. Soon after, however, he deleted the post.