Abelardo De La Espriella, a criminal defense lawyer with no previous political experience, appeared headed for a razor-thin victory on Sunday in Colombia’s presidential election, in what would be a win for his fervent supporters, the global right and President Trump, who had endorsed him.

Mr. De La Espriella — who transformed himself from sharply dressed Miami lawyer to populist in a soccer jersey and a straw hat — won nearly 49.7 percent of the vote with more than 99 percent of the votes counted, according to preliminary results from the agency overseeing the election. Iván Cepeda, a leftist senator and a longtime human rights advocate received nearly 48.7 percent.

His victory would return Colombia to conservative rule after four years under Gustavo Petro, the country’s first leftist president. It also advances Latin America’s broader shift to the right in Mr. Trump’s second term.

As word of the results spread, Bogotá, the capital, exploded with the din of cheers, car horns and vuvuzelas. People rushed into the streets. President Gustavo Petro said online that the results were preliminary and that there was “no president” until the votes had been scrutinized, which is the usual process — but cries of “Out with Petro” filled the air.

Mr. De La Espriella, 47, ran a high-voltage campaign complete with machine-generated flames; A.I. videos of tigers, his mascot; and pounding anti-Petro chants that made him something like a celebrity.

He also vowed to “disembowel” the left in Colombia and has asked the Trump administration to target his political opponents, leading critics to call him an autocrat in the making.