The dispute between President Luiz Inácio Lula da Silva (PT), who is seeking re-election and a fourth term, and Senator Flávio Bolsonaro (PL), son of former President Jair Bolsonaro, occurs amid a shift to the right in several Latin American countries.
A victory for Bolsonaro would represent the consolidation of this movement and Trump's regional influence, with the triumph of the right and a political ally of the Republican in the largest country in the region.
"It would be the fundamental piece of the so-called 'orange wave' (in reference to the American president) of leaders aligned with Trump that have been gaining strength and spreading across Latin America", political analyst Brian Winter, editor-in-chief of Americas Quarterly magazine, tells BBC News Brasil.
"Brazil is the biggest prize. It is personally important for Trump. He has a relationship with the Bolsonaro family that goes back many years," says Winter.
The analyst notes that, on the other hand, a victory for Lula would solidify the PT member's legacy as a historic figure on the left throughout the Global South. It would also create a "short-term headache" for Trump, given the relationship marked by "ups and downs."
"I think the White House would probably react with disappointment if Lula wins. And then it would seek some kind of pragmatic understanding with Lula," says Winter.
On Thursday (1st), three days before the first round, Trump stated in a conversation with journalists that he is following the election in Brazil "closely".
"Brazil is a very important country. It's a great country. The people of Brazil are fantastic. It's a very important election," said the American president, without mentioning any of the candidates.
On Friday (2), asked if he would support Bolsonaro, Trump again simply said that he was following the election closely. The statements were made amid the Brazilian government's fear that Trump will try to interfere in the Brazilian election.
Also on Friday, the United States and Australia announced the closure of their embassies and consulates in Brazil. Without detailing the reasons, the American government said that "the safety and security of American citizens is a top priority."
'Donroe' Doctrine
Since the beginning of his second term, in January last year, the American president has openly declared support for several right-wing candidates around the world and, specifically, in Latin America.
Recent examples include Abelardo De La Espriella, elected in June in Colombia, and Nasry Asfura, winner of last year's election in Honduras.
In Argentina, President Javier Milei's coalition (who took office in 2023, before Trump's return to the White House) won a major victory in last year's legislative elections after Trump suggested that financial aid to the country depended on the result at the polls.
Trump's public demonstrations of support came amid a recent wave of right-wing victories in Latin America.
Bolivia, Chile, Costa Rica, Ecuador and Peru are also among the countries in the region whose recent presidential elections were won by conservative candidates.
Swings to the right or left are considered common and occur periodically in Latin America.
Examples include the so-called "Pink Tide", as the shift to the left became known in the early 2000s, and the victories of leftist candidates in several countries, including Brazil, at the beginning of the current decade.
Furthermore, the current movement is also driven by anti-incumbency sentiment.
"I think it's less of an ideological issue and more of people being dissatisfied with the current leaders," Carlos Gustavo Poggio, a professor of International Relations at Berea College, in the American state of Kentucky, tells BBC News Brasil.
However, analysts emphasize that the current wave of right-wing victories occurs in a context of greater pressure from the United States on Latin America.
The Trump administration's measures in the region range from tariffs, such as those imposed on Brazilian products, to the capture of the president of Venezuela, the Chavista leader Nicolás Maduro, in January this year.
Poggio notes that the foreign policy vision espoused by Trump and his Secretary of State, Marco Rubio, is "a vision of hegemony in the Americas."
“I think Rubio is the main person (in the Trump administration) who is trying to put Latin America on the agenda,” adds political scientist Edgar Franco-Vivanco, a specialist in Latin American politics and professor at the University of Michigan.
The current American government's interpretation of the 1823 Monroe Doctrine, which defended American predominance on the continent, became known as the "Donroe Doctrine", in reference to Donald Trump's first name. The current administration advocates "restoring American prominence in the Western Hemisphere."
Security
A central issue in this scenario is public security, which is a priority both for voters in several countries in Latin America and for Trump's strategy on the continent.
Franco-Vivanco tells BBC News Brasil that many Latin American voters are disappointed with the failures of left-wing governments to control violence and now want to try something different. This opens up an advantage for right-wing candidates.
“At least in elections, the right is very good at saying they are going to do something about the problem,” he observes.
Among the right-wing leaders elected in recent years with promises to combat violence is Nayib Bukele, president of El Salvador, who has reduced crime in his country but is criticized for alleged human rights abuses.
Many of the right-wing politicians in Latin America aligned with Trump are open to greater security cooperation with the United States, including joint military operations.
The American government has been promoting a new "war on drugs" in the region and recently created the so-called Shield of the Americas, an initiative with the aim of expanding cooperation, security and the fight against drug trafficking, of which Brazil is not part.
The expectation is that, under a possible Bolsonaro government, Brazil would join the group.
The Trump administration has also designated cartels and criminal factions in the region as terrorist groups. The Brazilian groups Primeiro Comando da Capital (PCC) and Comando Vermelho (CV) were included in this list, despite opposition from the Lula government.
"On the American political agenda, whenever Latin America was mentioned, the issue of migration, security, drugs came into play. But, at the same time, it was never a priority in American foreign policy", highlights Poggio.
"The Trump administration seems to be placing Latin America as a priority. In this sense, things could take a new direction, because you have an administration that is promising, and has already carried out, military operations on the continent", he observes.
"The issue of classifying cartels as terrorist organizations, the justification for this is using military tools", says Poggio.
Tariffs and sanctions
Unlike several elections in other countries in the region, Trump has so far not openly declared support for any of the candidates in Brazil.
"Certainly, the Trump government prefers Bolsonaro to win this election, because he is a closer ally and there is greater political affinity between them. However, there has been no (so far) direct support", highlights Franco-Vivanco.
However, even without declared support, there is fear of attempted interference. Last year, the United States adopted measures against Brazil described as a response "to the witch hunts" suffered by former president Jair Bolsonaro.
Among the measures were tariffs against Brazilian products and sanctions against the country's authorities, including Minister Alexandre de Moraes, of the Federal Supreme Court (STF).
Recently, the newspaper Estado de S. Paulo revealed that, in negotiations at the time, one of the Trump administration's demands was that Brazil allow the participation of "political dissidents" in this year's election. The Brazilian side rejected the terms, remembering that there are no dissidents in the country, which is a democracy.
At the end of last year, tariffs and sanctions were eased. But this year the United States once again imposed new tariffs on Brazilian products, and authorities from both countries had visas denied or revoked.
After Lula's visit to Washington, his opponent, Flávio Bolsonaro, was also received by Trump at the White House. The announcement of the classification of the PCC and the CV as terrorist organizations, defended by Bolsonaro, came shortly after this visit.
In light of the Trump administration's actions since last year, Lula's response was to defend sovereignty, and research and analysts indicate that the measures may have benefited the Brazilian president more than his opponent.
"Any stronger statement from Trump could backfire", summarizes Poggio, remembering that, in several previous cases, the American president declared support for a certain candidate and ended up benefiting his opponent.
