Trump says Colombia's 'El Tigre' will be a 'great president' as socialist opponent launches legal challenge

With 99.9% of votes counted, de la Espriella led with 49.7% to Cepeda's 48.7%. De la Espriella, known to his supporters as "El Tigre," dominated in the country's mountainous interior and the vote-rich state of Antioquia, while Cepeda won in the capital Bogotá and performed well in coastal regions, following trends of recent presidential elections.

Cepeda has challenged the results, citing irregularities at thousands of polling stations. Nonetheless, overturning the election would be unprecedented in Colombian history.

If de la Espriella does hold out, it will mirror a continent-wide rightward shift seen in recent electoral results in Chile, Ecuador, Bolivia, and Peru, where conservative Keiko Fujimori appears poised to win the presidency.

Donald Trump congratulated de la Espriella during a signing at the White House on Monday. Trump told reporters that, "He called me last night, and he thanked me for the endorsement. He won. He won the election." In response to a question on relations between the U.S. and Colombia, Trump asserted that things would be "Much better. It'll be better. He's going to be a great president."

The election featured two candidates representing polar opposites of the Colombian political spectrum. De la Espriella, known as 'El Tigre' by his supporters, has enjoyed the enthusiastic backing of Donald Trump, promised a return to the law-and-order approach of former President Alvaro Uribe, and pledged an aggressive military campaign against guerrilla groups and criminal organizations, while Cepeda vowed to continue the negotiation-based strategy of Petro, a longtime political ally.

Ivan Cepeda is a longtime figure on the Colombian left, and served as senator for 12 years, following a four-year stint in the Chamber of Representatives. His father, Manuel Cepeda, was a prominent figure in the Colombian Communist Party, and was assassinated in 1994 during a particularly bloody era in Colombia's internal conflict.

On Sunday evening, Secretary of State Marco Rubio congratulated de la Espriella on the result, saying, "The Trump Administration looks forward to working closely with your incoming administration to advance regional security cooperation, end illegal immigration to the United States."

While the issues driving American and Colombian politics remain considerably distinct, de la Espriella's insurgent outsider campaign emulated Trump's in many ways, particularly in the sense that neither held elected nor appointed office prior to winning the presidency, launching successful campaigns almost entirely outside the existing party structure.

It is widely believed that Petro's negotiation-based approach and restraint with respect to military action has allowed such groups as the ELN (National Liberation Army), and various dissident elements of the FARC to regroup, boost recruitment and regain control of key territory and drug trafficking routes.

A de la Espriella administration will also likely mark a return to free-market economics, decreased governmental intervention in the economy, and a renewed push to lower taxes.

A strengthened U.S.-Colombia relationship is also a certainty, following an era of considerable tensions between Petro and Trump, which led to a series of acrimonious social media exchanges. Historically, Colombia was the U.S.' strongest ally in the region, but the relationship has weakened considerably under the tenure of Petro.

The Associated Press contributed to this report.