On Sunday, voters will return to the polls to decide between former Guatemalan first lady Sandra Torres and Alejandro Giammattei, a former head of the country's prison system. And it's an election that's proven to be nothing short of fascinating for many reasons.

The winner will lead not only Central America's most populous country, but also the nation which has become the leading source of migrants traveling to the US-Mexico border.

This means Guatemala's next president may find themselves on the receiving end of immense pressure from the US president and his administration. Limiting the number of migrants making it to the United States has been the centerpiece of Trump's policy agenda.

Guatemala's next president will be tasked with overseeing the final implementation of a controversial agreement reached between Trump and his Guatemalan counterpart, outgoing President Jimmy Morales, in late July. The deal has not yet been ratified by Guatemala.

Labeled as a "safe third agreement" by the White House, the deal would commit Guatemala to extending asylum to migrants who seek it when moving through the country. It would also make some asylum-seekers who pass through Guatemala ineligible for protection in the US. (Read the deal itself here .)

Critics say the preliminary arrangement endangers asylum seekers and may violate a court ruling stipulating that Morales cannot enter any "safe third" agreement without congressional approval.

When pressed Thursday on a time frame for the deal's implementation, US Customs and Border Patrol (CBP) commissioner Mark Morgan said: "We're still waiting on the government of Guatemala to ratify that. We're hoping anytime."

Donald Trump with the Guatemalan Interior Minister Enrique Degenhart and acting US Homeland Security Secretary Kevin McAleenan after signing the "Safe Third Country" deal in the Oval Office.

Just days before the deal was signed, Trump appeared to threaten Guatemala, saying: "We're looking at something very severe with respect to Guatemala."

This, along with other disparaging statements from Trump directed at the Guatemalan government, could be a sign that the biggest critic of the incoming administration may be nearly 3,000 miles away in Washington.

Former first lady vs. the "Eternal Candidate"

Both candidates for the Guatemalan presidency met with acting Homeland Security Secretary Kevin McAleenan last week to discuss the deal, according to the Washington Post . McAleenan has been visiting Guatemala in recent months as the two countries sought to reach an asylum agreement.

Torres and Giammattei have each questioned Morales' authority to sign the deal in the first place, with Torres, 63, tweeting the hashtag "NoAl3erPaisSeguro" (No to Safe Third Country).

Exijo @jimmymoralesgt que NO firme acuerdo con @POTUS que lesiona la soberanía que decía defender, ya que impone obligaciones económicas que no podemos asumir y desataría una crisis humanitaria.

¡Rotundamente nos oponemos a ser un 3er país seguro! #NoAl3erPaísSeguro — Sandra Torres (@SandraTorresGUA) July 13, 2019

"We ask President Morales to abstain from signing that agreement because we do not have many resources for our own people, much less can we keep foreigners here and give them attention," she wrote

Torres, from the center-left Unidad Nacional de Esperanza party (UNE), won 25% of the first vote in early June , nearly double the votes of her second-round rival. If elected, she would become the country's first female president -- but she brings political baggage with her.

Between 2008 and 2012, she served as Guatemala's first lady during the presidency of her husband, Alvaro Colom. She often acted beyond the remit of her ceremonial duties by assuming control of many of the country's social programs, says Marielos Chang, political scientist at the Universidad Francisco Marroquin in Guatemala City. She also attracted criticism due to a lack of transparency over funding.

Guatemala's former first lady Sandra Torres won the first round of presidential elections in June.

Giammattei, who won 14% of the primary vote , is a candidate for the center-right's Vamos party. He has been dubbed the "eternal candidate" because of his four attempts at the presidency, and is not without controversy himself. He notably faced accusations of being involved in the extrajudicial killing of a prisoner during his time as director of the prison service, according to Reuters . He was later absolved of any charges

He has described the outgoing Morales as irresponsible, but has not ruled out the asylum deal himself. "It should be the duty of the next government to conduct this negotiation," he said in a series of tweets criticizing the government and the terms of the deal.

"As a safe third country, we can't take care of our own, much less those from other countries. #ConcluGuateUSA," he said.

Making Guatemala a true safe third country

Guatemala's murder rate is five times that of the US, according to World Bank data from 2016 , the latest year available. Crime will also top the list of issues heavy on the minds of voters as they head to the polls this weekend, according to a Gallup poll conducted in July.

"We live in fear," Guatemalan Mario Macario told CNN Espanol . "Being out at night isn't the same anymore. We're left to wonder if we'll even make it back home safely." Human Rights Watch describes gang violence as a key factor driving Guatemalans out of their own country.

Alejandro Giammattei, known as the "eternal candidate," has run for the presidency four times.

The potential danger to vulnerable asylum seekers who would be sent to Guatemala under the new deal has triggered outrage from international rights groups. "Guatemala is not safe for Guatemalans," Meghan Lopez, from the International Rescue Committee in El Salvador, said in a statement to CNN.

"It is not safe for those who have fled seeking asylum from elsewhere in the Northern Triangle, and it is not safe for those who could now be returned to a country not their own - yet equally plagued by the same conditions they have fled."

Voters will also look for their next leader to cut down on corruption that has hampered Guatemala's ability to progress, after Morales ended the mandate of a United Nations-backed anti-corruption body . His presidency has been dogged by allegations of corruption ever since.

"Guatemala has had a long tradition of choosing between the 'least worst', but this election has definitely been the most complicated," says Chang. "There's no clear difference from who may be less harmful to the country."

Guatemalan migrants deported from the US standing outside an Air Force base after arriving in Guatemala City.

Like presidential elections around the world, Sunday's results may come down to the candidate's credibility as a change-maker, says Alejandro Ballsels, a political analyst in Guatemala City. "We're tired of protesting and seeing no results," Ballsels says, who compares the current political climate to a "boiling pot on the verge of exploding."

Perhaps the most imminent challenge for either candidate will be turning out the vote in the first place. Political experts have previously told CNN that voter turnout may drop from the 61% turnout seen during the primary

"We're at crossroads where participating politically will become almost an obligation, otherwise we will keep having daunting scenarios such as the one we will have this Sunday," warns Chang.