Colombia is heading for a run-off presidential election. Here's what's at stake
Petro, a former guerrilla himself, could find the vote difference with Duque too large to overcome, according to Paulo Gregoire, a Bogota-based analyst for geopolitical intelligence firm Stratfor.
"I think Petro will have a tough time because, to win the election, he would need over 90 percent of votes from Fajardo's supporters," Gregoire told CNBC over the phone.
In fact, Fajardo has already said he would not vote for either candidate.
Still, a poll by think tank the Latin American Strategic Center of Geopolitics held between May 29 and June 6 showed that Petro was now just 5.5 points behind Duque in the elections.
One of the main issues at play in the election is the peace agreement between the government and the rebel group, which is known as FARC.
Duque opposes the peace deal, claiming that it is too lenient on the former guerrilla members and has campaigned to modify the amnesty clause for them. On the other hand, Petro is supportive of the peace agreement, and is unlikely to change the terms.
The potential fallout of Duque's proposed modifications could result in more members of the rebel group mobilizing in the face of legal uncertainty. Former FARC fighters have already splintered off and created a new rebel group, the United Force of the Pacific. Increasing doubts over the peace agreement could push more people into the new rebellion.
"It is important to remember that around 300 to 500 FARC members chose not to join the initial peace agreement in the first place," Gregoire warned.
It would not be easy for Duque to modify the agreement even if he comes into power, however, as the deal is protected by the country's constitutional law. One way he could do it, according to Gregoire, is by calling for a referendum.
The two leading candidates would also move the country in opposite directions when dealing with the crisis in Venezuela. Approximately 1 million Venezuelans have entered Colombia since the country fell into an economic crisis in 2017.
Duque's campaign asks Colombian voters to "Vote so that Colombia does not become another Venezuela," and Gregoire said that Duque would take a tougher stance against the neighboring country in terms of sanctions, in the form of backing the U.S.-imposed penalties on the Venezuelan oil industry.
A Duque victory in the elections — as appears probable — would likely see the relationship between the two countries become even worse, Gregoire said. A Petro victory, however, would probably mean Colombia employs softer approaches regarding Venezuela.
The Stratfor expert said a significant portion of Colombia is wary of Petro's historically close ties to Hugo Chavez, however, and they fear a leftist presidency will lead to their country becoming similar to Venezuela.
Petro has become more critical of Venezuelan President Nicolas Maduro recently, but, in any case, his foreign policy toward Venezuela is likely to be much more passive than Duque, Juan Felipe Celia, assistant director and Colombia lead at think tank the Atlantic Council told CNBC.
In terms of the Colombian economy, Petro is advocating for less reliance on oil and exports. He also says he wants to tackle big business conglomerates in the country. For example, he wants to purchase land from some major agriculture producers to redistribute to smaller farmers. Meanwhile, Duque wants to increase oil production through exploring other technology such as fracking.
Duque represents a more business-oriented model that would attract more investor confidence while Petro has highlighted social policies, which may not directly benefit the economy, Celia told CNBC.
"Duque is more traditional and he is supportive of foreign investments, lowering taxes and spurring investments," Celia said, "Petro brings about uncertainty as he proposes a move away from traditional avenues. He is also not necessarily business friendly in his rhetoric."