BOGOTÁ, Colombia—For decades, Colombia elected establishment politicians who hewed to the center-right and helped make the country the U.S.’s closest ally in the region.

That era has come to a halt. In Sunday’s presidential election, voters will choose between two antiestablishment politicians. Gustavo Petro, a 62-year-old senator, is a leftist former guerrilla who says he wants to end oil exploration and print money to spur economic growth. Rodolfo Hernández, his rival, is a 77-year-old provincial real-estate mogul who didn’t hold rallies or speak from public squares. He reached voters through homemade TikTok videos, in which he calls himself “the Old Guy with Sass.”

Colombia joins countries throughout the region—including Mexico, Chile and El Salvador—in which antiestablishment politicians have pushed aside existing parties and demolished the political center.

“The forces that operated in the center of the political spectrum have perished,” said Humberto de la Calle, a Colombian congressman and former government minister. He once belonged to the Liberal Party, which, along with the Conservative Party, had dominated politics since the 19th century, but now shares the legislature with 20 other parties and movements.

“The result of that extreme fragmentation of the parties has been a breeding ground for those who are the most radical and who frequently use populist language,” Mr. de la Calle, now a member of the Green Oxygen party, said.

While the antiestablishment shift is happening elsewhere in the world, it is particularly pronounced in Latin America. The Colombian election marks the region’s 14th consecutive presidential vote in a democracy in which the incumbent either lost or was unable to marshal the support for a successor to win, said Oliver Stuenkel, who tracks politics for the Getulio Vargas Foundation, a think tank in São Paulo.

Voters from Mexico City to Manaus are frustrated that governments have been unable to address three key issues, he said: low economic growth, income inequality and poor public services. All were made worse by the recent pandemic. “These three are so corrosive to society, they undermine what it means to be a society,” Mr. Stuenkel said.

In Colombia, Mr. Hernández was a relatively little-known figure until this year. Besides his career in real estate, he had a three-year stint as mayor of Bucaramanga, a city of 600,000. In that time, he was suspended by the country’s Inspector General’s office twice, for hitting a councilman and verbally abusing a citizen. He faces a criminal charge for allegedly rigging a contract favoring a waste-disposal company. He denies the allegations and calls them politically motivated. If elected, he would be the first Colombian to win office under indictment.

His folksy videos slamming the political class in Colombia and calling for an honest government have drawn millions of views, and helped propel him from single-digit support just weeks ago to a small lead in some polls over Mr. Petro ahead of Sunday’s vote.

Supporters of Mr. Hernández gathered in Bogotá after he placed second in the first round of presidential election voting last month.

Photo: LUISA GONZALEZ/REUTERS

Mr. Petro, a senator known for clashing with conservatives in Congress, pledges to phase out oil production and transition to a state-centric model in which he would provide jobs for the unemployed, make universities free and give credit and land titles to poor farmers.

In the election’s first round last month, election authorities reported that Mr. Petro won 40.3% of the vote in a field of six candidates that included Federico Gutiérrez, a 47-year-old former mayor of Medellín who had the support of incumbent President Iván Duque’s conservative party. (Mr. Duque was ineligible for a second term.) Mr. Hernández had the second-highest number of votes, 28.1%, in the election, in which 21 million of the country’s 39 million registered voters cast a ballot. With neither candidate commanding at least 50% of votes, the two face a runoff election Sunday.

Slow growth

The political turmoil has spread across Latin America as governments deal with corruption scandals, high crime and sluggish economies. Annual economic growth from 2013 through 2019 was 1.2% on average for the region, World Bank data shows.

It followed a decade when the region’s young democracies rode high on the rising prices of its commodity exports like oil, soy and metals. Incumbent politicians notched years of easy victories as state coffers swelled. However, the economies of the region failed to diversify, and the end of the commodities boom brought fresh malaise.

Steven Levitsky, director of the David Rockefeller Center for Latin American Studies at Harvard University, said the region is returning to its previous state of high public discontent with weak nations that have failed to build strong institutions and that struggle to deliver on basic services, from decent public health and education to crime-free streets.

Latin America suffered the world’s biggest economic contraction in 2020, when the Covid-19 pandemic and restrictions wreaked havoc on growth and public debt and killed more than 1.6 million people. Growth bounced back last year with a 6.9% rise, but the International Monetary Fund estimates that per capita incomes in the region by 2025 will be the same as in 2015.

Poverty dropped from 45.6% in 2003 to 27.8% in 2014, but has gone up since then, hitting 32.1% in 2021, according to the United Nations Economic Commission for Latin America and the Caribbean. Extreme poverty in 2021, at 13.8%, was the highest in 27 years. Fast-rising inflation and high unemployment in the wake of the pandemic has hit the region’s vulnerable middle class, economists and analysts say.

Latinobarómetro, a Chilean polling firm, captured the public disenchantment in their latest survey on democracy and government across Latin America. Confidence in government was down to 27% in 2020 from 43% in 2010. While support for authoritarianism was minimal, only 25% said they were satisfied with democracy, down from 44% in 2010. Seventy percent were unsatisfied in 2020.

“The people, what they want is that the political system resolve their everyday problems, employment, social security, healthcare,” said María Fernanda Espinosa, a former president of the U.N. General Assembly who had previously served as defense and foreign minister in Ecuadorean governments. “That’s why they have these outsiders winning.”

In Chile, long admired in the region for its sophisticated and consensus-seeking political class, a leftist former student protest leader won the presidential election in December over a conservative lawyer. The candidate supported by the sitting president hadn’t made it to the second round after being defeated in the first by another antiestablishment rival who campaigned from the U.S.

Then-presidential candidate Pedro Castillo of Peru spoke at a rally in Lima last year.

Photo: Raul Sifuentes/Getty Images

In Peru, Pedro Castillo, a rural schoolteacher and union activist from a little-known party who had no political experience, took the presidency in June of last year after winning just 15% of the ballots in a first round of voting. He has since faced two impeachment votes, changed his prime minister four times and is facing a criminal investigation for corruption, an allegation that he denies. He has a disapproval rating of 70%, according to polling firm Ipsos.

In El Salvador in 2019, Nayib Bukele, a former marketing executive who created his own political party, became the first president elected since the 1980s who didn’t come from one of two established parties.

The region’s two biggest countries, Mexico and Brazil, are now run by populists. The political party that backs Mexico’s leader Andrés Manuel López Obrador was created in 2011 and had no governorships in 2018, the year Mr. López Obrador won election. After the latest gubernatorial elections, it will govern 22 of the country’s 30 states. In Brazil, which has few guidelines to limit the creation of parties, there are now 32 registered parties, with 24 in Congress.

Backlash

Many of the recent election winners in the region have been leftist candidates. If Mr. Petro were to win in Colombia, that would leave Brazil as the region’s sole big country to be governed by the right, at least for now. Polls there show former leftist President Luiz Inácio Lula da Silva well ahead of incumbent Jair Bolsonaro before October’s vote.

The antiestablishment shift has less to do with ideology and more with a backlash against traditional parties of all stripes, political analysts say. Many of the new crop of politicians can be hard to define on a political scale. Mexico’s leftist president is a social and fiscal conservative, while the right-wing Mr. Bolsonaro in Brazil has ramped up public debt.

Mr. Petro’s supporters followed election results in Bogotá last month.

Photo: Robert Bonet/NurPhoto/Zuma Press

Colombia’s Mr. Hernández says he would trim bloated budgets, but he also pledges to pursue a peace deal with leftist rebels, provide more aid for students to attend university and modernize a poor countryside.

“That old man says he thinks about the people,” said Alexandra Holguin, who earns under $200 a month as a maid and says she struggles to make ends meet. “I like it that politicians aren’t running him, manipulating him. What’s good about the old man is he’s independent.”

In Argentina, hobbled by 60% annual inflation and growing poverty, Maria Colman Rodriguez talks of how she struggles to raise three children in a poor barrio outside Buenos Aires. She’s tired of the ruling party, the Peronists, but doesn’t much like the others, either.

“No politician helps us, I’m disappointed with all of them,” she said. “When they get to power, they forget about the people. Everything’s too expensive. My money doesn’t cover what we need to make it to the end of the month.”

She’s starting to take a look at Javier Milei, a long-haired 51-year-old who rails against the ruling class in televised interviews and leads the polls for president ahead of next year’s elections. In a phone interview, Mr. Milei called himself an anarcho-capitalist who would close ministries, dramatically cut back spending, cut taxes and close the central bank, which he said fuels inflation.

“The only thing politicians do is tell the people what they want to hear,” said Mr. Milei. “Let them keep voting for that trash. I don’t care. I say what I think, I act and work along my liberal principles, no matter who likes it or not.”

Much of the new generation of antiestablishment leaders won power without rising through the ranks of parties and using traditional media. Their communication is heavy on TikTok videos, Twitter and Facebook Live—and aimed at the young and those disenchanted with the status quo.

Javier Milei on stage at the Buenos Aires International Book Fair last month.

Photo: luis robayo/Agence France-Presse/Getty Images

In one TikTok video for Mr. Hernández, two cartoon figures begging for money flank him as he criticizes those who say he’s rightist or leftist. “All this talk about right, left, center—it’s all lies. The hunger, the neglect, the lack of good education, is that right or left?” he says. Another features his 97-year-old mother, bobbing her head as a reggaeton artist sings to the lyrics, “He’s going to do away with the criminals,” in reference to corrupt politicians.

“Social media has changed the world, and now this is the way you can choose the leader of a country,” said Gabriela Acevedo, a 23-year-old follower of Mr. Hernández in Colombia who ended up joining the communications team that helped the candidate quickly become a household name. “Through the social networks you can use photos or a video or say something funny and connect with the people. It’s a way to make the citizen and the candidate one.”

There’s a risk the new generation of outsider and populist leaders—who often seek to govern through the force of their personality—will fail to build more competent governments and could sideline institutions like congress and the judiciary rather than strengthening them, said Harvard’s Mr. Levitsky.

In Brazil, Mr. Bolsonaro has repeatedly cast doubt on the country’s election process, stoking fears among some analysts that he might try to disregard a losing outcome in October. Mexico’s president has disbanded the country’s federal police and hollowed out independent regulatory agencies.

El Salvador’s president, the 40-year-old Mr. Bukele, has been criticized by the U.S. government for a power grab that included the removal of judges and stacking the courts with his supporters. Over the past two months, his administration has arrested more than 40,000 people with alleged links to criminal gangs after constitutional guarantees were suspended to curb a rise in homicides. Rights groups say that the measures have led to arbitrary arrests and weakened the rule of law.

The traditional parties left behind across the region are often like Peru’s Popular American Revolutionary Alliance, or Apra. It no longer has anyone in Congress. One of its former longtime congressmen, Jorge del Castillo, misses the time when the Apra filled congress with lawmakers, ran its top leaders for the presidency and churned out technocrats for government jobs. Gone are the days, he said, when parties had ideologies, debated issues of the day and provided stability. “Peru’s in the hands of people who improvise,” he said.

—Jenny Carolina Gonzalez and Silvina Frydlewsky contributed to this article.

Write to Juan Forero at Juan.Forero@wsj.com and David Luhnow at david.luhnow@wsj.com