The election of a former Marxist guerrilla as president of Colombia, along with the brightening political fortunes of fellow leftist Luiz Inácio da Silva in Brazil, has observers of the region speculating about the possibility of a bloc hostile to American interests emerging in South America.

Gustavo Petro on Sunday defeated a right wing populist businessman, Rodolfo Hernandez, in a runoff election for the Colombian presidency. His victory means all of South America’s major countries with the exception of Brazil will be governed by leftists.

Currently governed by a rightist who is an ally of President Trump, Jair Bolsonaro, Brazil could be the next domino to fall. If that happens, Mr. Petro may seek to form a “strategic front against the United States,” a Venezuelan political consultant, Pablo Quintero, said.

“Petro is waiting for [da Silva] to win this upcoming election, and for Nicolas Maduro to win in 2024 to reinforce the region,” Mr. Quintero said, referring to the socialist president of neighboring Venezuela. 

“Almost every president of Latin America called me, and I can finally propose a dialogue in the hemisphere, without excluding anyone,” Mr. Petro said in his victory speech. “It’s time to sit down with the government of the United States and talk about what affects everyone in the hemisphere.”

Mr. Petro, who fought for years against the Colombian government as part of the M-19 guerrillas before turning to politics, won the election with promises of agrarian reform, combating climate change, and ending the country’s long reliance on crude oil and coal exports. He also has pledged to end domestic oil exploration.

In 2021, Colombia was South America’s second-largest petroleum producer, after Brazil, and the fifth-largest crude oil exporter to the United States, according to the United States Energy Information Administration.

Some analysts believe that if Mr. Petro follows through on his threat to stop exploring for oil, the country’s reserves will deplete rapidly — probably within six years. 

A professor of Latin American history at Florida International University, Victor Uribe, said such a move would have “a very serious economic impact on Colombia.”

“If Colombia transitions away from exploring oil — even if it doesn’t immediately stop — in six years there is not going to be a viable alternative to even produce the oil required for internal consumption,” Mr. Uribe said.

He also sees a high likelihood of the ideologically aligned countries of South America putting up what he called a “common front to undertake conversations with the United States.”

To do that, the group must first unseat Mr. Bolsonaro, who faces what looks like an uphill battle against Mr. da Silva, the leftist candidate popularly known as Lula. A recent poll by Brazil’s Estadão newspaper put Lula ahead of Mr. Bolsanaro by 17 points.

When he announced his candidacy on May 9, Mr. da Silva told supporters that Brazil’s situation “forces us to overcome our differences and build an alternative path to the incompetence and authoritarianism that governs us.”

Mr. da Silva was president of Brazil between 2003 and 2010, a period of strong economic growth. After leaving office, he was convicted of money laundering in a case known as “Operation Car Wash,” in which executives at the national oil company, Petrobras, were alleged to have accepted bribes from construction firms in return for awarding them contracts at inflated prices. 

Mr. da Silva was convicted and sentenced to a 12-year prison term, but Brazil’s Supreme Court annulled the conviction last year after determining that the judge who headed the investigation, Sergio Moro, was biased and abused Mr. da Silva’s right to a fair trial.

The presence of ideological allies on its border could improve Mr. da Silva’s chances even more, Mr. Quintero said, and should not be ignored by the United States.

“I think there’s a reorientation to the left that needs to be listened to,” he said.