A leading presidential candidate in Colombia’s upcoming election is taking a cue from President Trump and claiming that the vote will be rigged against him even before the polls open on Sunday.

In an interview with Spain’s El Pais newspaper, Gustavo Petro, a former Marxist guerrilla fighter, accused the institution that directs the country’s elections of being biased in favor of his opponent and of handling votes in a “suspicious” way.

“It has a clear affinity with the other candidate,” Mr. Petro said. “I was talking to people at the election institution, and the meeting was interrupted because the other candidate arrived. They suspended my conversation to go meet with him.”

Mr. Petro said that during the primaries last month, his team “detected alterations” in the institution’s software and discovered that 700,000 votes went missing. 

“We were able to get them back, but we had to claim them,” Mr. Petro said. “There is a huge risk.” 

Sunday’s election is being billed as a watershed for both Colombia and the hemisphere. If Mr. Petro defeats his conservative opponent, Rodolfo Hernandez, it would mark Colombia’s first decision to embrace a president from that side of the political spectrum. All of South America’s major countries would then be governed by leftist leaders.

Regardless of the outcome, Colombia will be electing its first anti-establishment president, a political analyst, Pedro Viveros, said.

“This is the first time there are two candidates, each with their style, who believe that they have a genuine base of popular support,” he said. “This is the first time in Colombia that we will have a populist president, of the left or of the right.”

The most recent polls show a tight race. A social research and communication consulting firm, GAD3, predicts Mr. Petro will win by less than two points; a poll from Guarumo and EcoAnalítica gives Mr. Hernandez the edge by about two points. 

Mr. Petro is calling for a new economic model for Colombia, one that halts oil exportation and breaks the country’s long reliance on crude oil and coal exports. He also wants to raise taxes on the 4,000 wealthiest Colombians and promised a public sector job to anyone who is unemployed. 

During the final years of Colombia’s 52-year-old civil war, Mr. Petro fought with the Marxist guerrillas of the 19th of April Movement, a group that emerged in 1974 and later came to be known as M-19. It was the second-largest guerrilla group in the country, following the Revolutionary Armed Forces of Colombia, known as FARC.

The war essentially ended in 2016 when most of the rebels agreed to a cease-fire. Some of these fighters, including Mr. Petro, entered politics.  

Mr. Hernandez is a 77-year-old businessman who made his fortune by building low-income houses in the 1990s. He was the mayor of Bucaramanga until 2019, and has campaigned against corruption and promoted himself — often through his popular TikTok account — as a person who can transform Colombia.

A professor of Latin American Colonial and Modern History at Florida International University, Victor Uribe, predicts that Mr. Hernandez will win the race despite Mr. Petro’s success in the primaries.

“I even venture to say that he is going to win not by a small margin — as analysts and polls are predicting — but by as many as 1 million votes or more,” Mr. Uribe said.

While Mr. Petro promises to call out election fraud if his team finds it, he claims he would work to prevent any social unrest related to the outcome. Unlike Mr. Trump, he says he will ultimately respect the results.

“If we say there’s fraud it is because we saw it, and we verified it,” he pledged.  “If a transparent result is adverse to me, I will accept it. Same thing if I win, but it has to be transparent.”