A group of 31 Democratic members of the US Congress has sent a letter to the secretary of state, Marco Rubio, criticising the Trump administration over “an apparent effort to interfere with, and potentially undermine” Brazil’s elections.

Latin America’s largest country will hold the first round of its presidential election on 4 October, with polls showing a technical tie between the incumbent leftist president, Luiz Inácio Lula da Silva, and the far-right senator Flávio Bolsonaro.

In the letter sent to Rubio on Thursday, 30 members of the House of Representatives and the senator Bernie Sanders listed a series of measures the US government took that they interpret as attempts to secure a victory for Flávio Bolsonaro, the son of former president Jair Bolsonaro.

They wrote that those actions “reflect a broader and deeply troubling pattern of interference by the Trump administration in recent electoral processes across Latin America”, citing the recent midterm elections in Argentina and presidential elections in Honduras and Colombia.

In July, a group of 11 Democratic members of Congress sent a similar letter arguing that Trump was trying to interfere in Colombia’s election in favour of the far-right millionaire and former lawyer Abelardo de la Espriella, who went on to win the second round.

In Thursday’s letter, the lawmakers wrote that US officials under Trump have engaged in a “sustained, multifaceted campaign of interference and destabilization in Brazil”.

They noted that when Trump imposed his first tariffs on Brazil last year, he made clear that one reason was Bolsonaro’s trial over his attempt to stage a coup after losing the 2022 election to Lula. The former president was ultimately convicted and, unable to run because he is under house arrest, named his son to take his place.

They also cited new tariffs that Trump imposed on Brazil in July under section 301 of US trade law, as well as plans by the US government to send two officials to Brazil to “cast doubt on the integrity of the country’s voting system, considered by many experts to be among the most reliable in the world”. The Lula administration ultimately denied the US delegation visas.

The Brazilian president has repeatedly complained that Trump is trying to interfere in Brazil’s elections and, in his speech opening the UN general assembly this week, said, without naming anyone: “We will not allow the enemies of democracy, internal or external, to act against the will of the people.”

In the letter sent to Rubio on Friday, the Democratic lawmakers wrote that “rather than helping defend Brazilian democracy”, the US is “acting as an agent of interference and destabilization – using diplomatic, economic and political pressure in ways that risk weakening the country’s democratic institutions, undermining confidence in Brazil’s electoral process, and inflicting further damage on our relationship with a strategic US ally and trading partner”.

They asked Rubio if Trump’s administration would commit to respecting Brazil’s electoral process and requested a response by 1 October.

The state department has not yet responded to a request for comment.