By Kejal Vyas and Juan Forero

BUCARAMANGA, Colombia—Presidential candidate Rodolfo Hernández doesn’t hold rallies or take part in debates. Instead, the 77-year-old construction tycoon and self-proclaimed King of TikTok campaigns through social-media videos recorded from his kitchen and yard, where he rails against politicians he says are crooked and promises to jail them without mercy.

With off-the-cuff zingers aimed at his adversaries, Mr. Hernández’s low-budget, folksy approach to campaigning has helped him rise from obscurity in a matter of weeks to what polls show is a statistical tie ahead of a June 19 runoff election with self-described leftist contender Gustavo Petro, who wants greater state control of the economy and credits for poor farmers.

“I won’t steal, which is at the root of the evil in Colombia. I won’t lie, I won’t betray voters,” Mr. Hernández said Monday on Facebook Live, one of his favorite methods of reaching Colombians. “With one small modification to the penal code, all those crooks who have robbed Colombia and deceived Colombians will be truly punished and jailed.”

Such frank talk—posted on social media by a team of 20-somethings who are the backbone of Mr. Hernández’s campaign—propelled him past four other candidates in the recent first round of voting even though he has no political party or, until recently, national name recognition.

Positioning himself as an outsider in a country wary of establishment politicians, Mr. Hernández could be an election away from leading the U.S.’s closest ally in the region, the third-most-populous country in Latin America and a nation vital in the fight against the cocaine trade.

For years, this country of 50 million has been dominated by conservative former President Álvaro Uribe and well-established parties that hewed to traditional campaigning, hiring foreign advisers and spending big on ads.

Colombia’s election, between Mr. Hernández and Mr. Petro, who in the 1970s and 1980s was in a now-defunct revolutionary insurgent group before becoming a senator, mirrors the antiestablishment tide that has swept across Latin America in recent years. It has resulted in political iconoclasts winning office from Mexico to El Salvador, Peru to Chile, while ousting traditional parties. Growing poverty and unemployment triggered by the pandemic have deepened the disenchantment with the ruling elite.

“The people are tired of the political constructs, party structures, party bosses, big plaza gatherings,” said Lawrence J. Gumbiner, a former U.S. diplomat who advised Mr. Hernández on international relations.

Mr. Petro, in public comments this week, said Mr. Hernández’s proposals are similar to his and could lead to a national accord between the two political movements in the future, without offering more detail.

Calling himself “The Old Guy with Sass” on TikTok, a video app popular with young people, Mr. Hernández can be seen every other day. In spliced videos with psychedelic colors and sound effects, he laughs at his critics and lifts weights, wearing a tracksuit.

Victor López, who advised the campaign until January, said the focus was on getting Mr. Hernández’s face onto Colombians’ smartphones because research showed 80% of Colombians use social media. And drawing on the candidate’s sense of humor has been a big part of the strategy, said Mr. López.

“Am I too old to be on TikTok?” Mr. Hernández says in a video to his nearly 600,000 followers, flanked by young supporters who start dancing to the words, “I don’t care.”

As he has been catapulted into prominence, the antiestablishment message that Mr. Hernández has cultivated in his posts is coming under attack as details of his life as a developer and then mayor of this city of 600,000 come to light.

The Attorney General’s Office said it has indicted him for allegedly trying to rig a contract for trash collection to benefit a company when he was mayor and called him in for questioning on July 21, just 16 days before the scheduled presidential inauguration. Another entity, the Inspector General’s Office, has 33 open investigations of him on allegations ranging from workplace harassment to defamation to contract irregularities.

Three disciplinary sanctions that the inspector general ordered against him forced him out of office as a councilman and led to his suspension for several months as mayor. One of them was for his 2018 assault of a city councilman, which was captured on video and went viral during the campaign.

“He’s a right-wing populist who doesn’t represent deep change. His discourse lacks depth, and he says what people want to hear,” said John Claro, the councilman captured on video being slapped by Mr. Hernández.

With the vote two weeks away, Mr. Hernández’s team said they were overwhelmed and couldn’t make him immediately available for an interview. His aides didn’t address questions about his record, the indictment, the investigations and the sanctions against him.

In public statements, Mr. Hernández has characterized the assertions against him as political.

“They say I have criminal processes against me, but they were assembled by the political insiders when I was mayor of Bucaramanga,” he told his followers via Facebook on Monday. “Not just one but 200 when you take into account administrative and criminal. My only alliance—and take a good look at my face—is with the Colombian people.”

In an interview this past week on La W Radio, he took on the most serious of the allegations—attempting to rig a contract favoring one waste-disposal company—and called it a reprisal for removing spending powers from City Council members and cutting costs that he says helped him close Bucaramanga’s budget deficit.

“How am I not going to be indicted when there was this bunch wanting to kick me out of the mayor’s office,” Mr. Hernández said. “What was the motive? To get me out because I took away their checkbooks. There’s no one who’s had more investigations than me.”

If he wins, Mr. Hernández would be the first Colombian president-elect in recent memory who has been indicted, according to Colombian legal experts. A criminal case against a president would likely be passed from the municipal court where it is being handled to a congressional legal committee, which would decide its merit, according to Camilo Burbano, law professor at Bogotá’s Externado University.

Mr. Hernández regularly reminds viewers of his past as a successful businessman and job creator. Unlike his rivals, he assures them, he isn’t a politician.

“Around the world, people are seeing politicians as part of the problem rather than the solution,” said Mr. López, the former adviser and the founder of the political strategy firm Kayros Group.

Henry Rodríguez, a factory worker just outside Bogotá, said he would vote for Mr. Hernández because of his hard critiques of politicians. He called the candidate a fresh face and didn’t mind voting for someone he barely knows.

“As we say here, if you’re not willing to bet on an egg, you’ll never get a chicken,” Mr. Rodríguez said.

Another supporter, Viviana Morales, a Bogotá architect who is unemployed, said Mr. Hernández talks clearly about what she calls good options for this country.

“He speaks in a colloquial way that any person can understand, and that generates confidence in me,“ she said. ”You know what he’s saying, and that’s enough.”

Supporters of Mr. Hernández’s opponent say Mr. Petro is the only true candidate for change. As a longtime opposition lawmaker, Mr. Petro has come up with ideas to centralize economic management and redistribute wealth that have never been given a chance by the political elite, said pensioner Jose Luis Bautista. “Petro’s the only one who cares for the poor,” he said.

Mr. Petro, has energized young people with a pledge to fight climate change and end Colombia’s reliance on coal and oil. His promise to boost agriculture by showering poor farmers with subsidies and credits plays well in this country’s vast rural zones. He, too, rails against corruption and the establishment, which he asserts has made Colombia poor and violent.

Mr. Petro’s message that his opponents were entrenched, right-wing oligarchs opposed to change doesn’t work with Mr. Hernández, who is harder to pinpoint ideologically. Indeed, some of Mr. Hernández’s proposals are popular among liberal voters—and not much different from his adversary’s.

He vowed to curtail riot police and provide more aid so more students could attend university. He said he would renew relations with Venezuela’s authoritarian regime, which the U.S. has tried to isolate since 2019. He pledges to modernize poor rural regions.

Like his rival, Mr. Hernández has said he would pursue peace talks with a drug-trafficking rebel group, the National Liberation Army, which in 2004 kidnapped his daughter, Juliana. Mr. Hernández said he refused to pay a $2 million ransom. Juliana was never seen again and is presumed to have been killed.

One promise has raised eyebrows: He would see to it that Colombians take vacations.

“All Colombian families should have the right to get to know the ocean, at least once in their life,” he said, providing scant detail about how he would pay for the pledge. “In my government, I’ll work so that right becomes a reality.”

Mr. Hernández’s speeches at times bear striking similarities to those of the populist leaders that he lauds.

“We can’t have a rich government while the people are poor,” he said recently, the same line used in a past address by Mexican President Andrés Manuel López Obrador, whom Mr. Hernández admires.

In policies, too, he has taken a page from Mr. López Obrador. Mr. Hernández said he would donate his salary for scholarships and cut 90% of the expenses of the presidential residence by converting it into a museum and getting rid of the palace chefs. He said he wouldn’t take support from other politicians, leaving analysts to wonder how he would get legislation approved with no political party or few allies in Congress.

In Bucaramanga, some of Mr. Hernández’s former colleagues say they don’t trust his antigraft rhetoric.

Carlos Sotomonte, an environmental activist, chaired Mr. Hernández’s political movement known as the League of Anticorruption Leaders, which carried him to the presidential race.

Mr. Sotomonte said he quit after a year because of Mr. Hernández’s style and because he said the candidate maintained business relationships with politicians in his state even as he publicly lambasted them as corrupt.

“I think he’s dangerous, I’m afraid of him,” said Mr. Sotomonte, who is supporting Mr. Petro. “The only reason Rodolfo has captured that antiestablishment vote is because people don’t know him.”

In neighborhoods where Mr. Hernández once made low-cost homes during his time as a builder, residents complained about shoddy construction made on land not conducive to developments.

Neither Mr. Hernández nor his campaign responded to questions about the projects.

The projects in two locations here involved homes built with tax breaks and other benefits given to the builder, according to court papers. At the time—in the 1990s—Mr. Hernández was an established developer who presented an image of a socially conscious builder interested in the welfare of home buyers. Lawsuits against him—and a court order for him to move residents to safer ground in one location—showed that his projects had run into problems ranging from floors that had collapsed to houses that had sunk in uneven terrain.

“He fooled us,” said Doris Gomez, whose home started to collapse in one development, forcing her to move.

Some people who worked with him describe a leader who as mayor demanded that policies be put in place immediately, sidestepping government regulations and verbally abusing those who pressed back.

Elmer Rojas, an agent with Bucaramanga’s transit-regulation force, recalled how Mr. Hernández hurled invectives at agents when he summoned them for a meeting, a video of which Colombians have shared on social media.

“He said we were the face of the city and he told us we were all corrupt,” said Mr. Rojas. “He said he’d fire us and to try to sue him if we didn’t like it.”

Neither Mr. Hernández nor his campaign responded to questions about the incident.

In his moments on social media and in meetings with journalists, Mr. Hernández strikes a more amicable tone. But there have been gaffes.

In a 2016 radio interview, he claimed to be “an admirer of a great German thinker: Adolf Hitler.” He later apologized and said he had meant Albert Einstein.

Jenny Carolina Gonzalez contributed to this article.

Write to Kejal Vyas at kejal.vyas@wsj.com and Juan Forero at Juan.Forero@wsj.com

wsj.com 06 04 2022