Since taking office, President Trump has become an electoral kingmaker in Latin America, with right-wing candidates aligned with Washington winning the past seven elections in the region. Mr. Trump has explicitly endorsed winners in two of those contests.

Now comes the biggest regional test for the right and for Mr. Trump's record of success: Brazil. As Brazilians prepare to head to the polls, the question looming over Latin America's largest democracy is whether Mr. Trump will try to influence its presidential election next month. The American leader has not openly championed any Brazilian candidate, but the Trump administration has in more subtle ways lifted the right-wing challenger, Flávio Bolsonaro, who is running against President Luiz Inácio Lula da Silva, a leftist.

Brazil is erecting a firewall to try to shield its high-stakes vote from potential U.S. meddling. The efforts include monitoring foreign lobbying and disinformation, fighting false claims about electoral fraud and enlisting the support of countries in Europe, Asia and elsewhere.

The concern for many in Brazil is that, in a narrow contest where polls show Mr. Bolsonaro and Mr. Lula practically tied, a helping hand from Mr. Trump could tip the scales in the challenger's favor.