Nogales, Mexico — Ivan Castro Santos, his wife and their four children, including 1-year-old triplets, have been living in a crowded room over the past four months, waiting for an opportunity to enter the U.S.

The young family traveled from southern Mexico to the House of Mercy and All Nations shelter in the northern Mexican border city of Nogales, joining other migrants from across Latin America. Half of them are children. All of them have one objective: finding work and safety in the U.S.

Castro Santos, 22, said he and his wife, Fatima Gonzalez Hernandez, 19, decided to leave Guerrero, Mexico, because of "the crime and the risk to the children" there. "To protect them," he added in Spanish, looking at his young children.

Tens of thousands of migrants are estimated to be in Mexico, hoping to enter America, including through a program that allows them to use a smartphone app to request a time to be vetted, processed and admitted by American border officials. The system was established by the Biden administration to dissuade migrants from crossing the border illegally, but many are now worried that President-elect Donald Trump will make it much harder for them to make it into the U.S. at all.

Castro Santos said he's worried about Trump "canceling the appointments" offered by the U.S. government app, known as CBP One. "We don't want to run that risk of going back and putting them at risk," he said, referring to his children. If allowed into the U.S., he said his family would like to settle in Houston, where his sister lives. He said he would like to learn how to cook and work in a restaurant.

Trump made tackling illegal immigration a central theme of his campaign, running on a platform of mass deportations, harsher asylum rules and a reversal of the Biden administration's border policies, including the app-powered entry system used by migrants in Mexico. His immigration promises appealed to many American voters, polls show, including those living near the southern border.

Anna Parada, who was born and raised in Nogales, Arizona, just miles away from the border with Mexico, said the "main" reason she voted for Trump was because of his stance on immigration.

"I really saw the Biden administration being a little bit too lax on immigration," Parada said. "And having Trump back in office, I believe it's going to be a difference again."

On the Mexican side of the border, the reaction to Trump winning was dramatically different.

Luz Angela, a migrant from Bolivia, said she felt "scared" when she learned American voters had elected Trump.

"I felt scared because he promised in his speeches that they would deport all the migrants," Angela said in Spanish. "And that he would close the CBP One application."

A doctor by trade, Angela said she and her 9-year-old son, Matias, fled political persecution in Bolivia. She said she was targeted by the government there after complaining about corruption in the hospital where she worked.

Angela and her son have been waiting for a CBP One appointment for nearly 7 months since arriving in Nogales, Mexico. During her wait, she has volunteered as a doctor at the House of Mercy and All Nations shelter, treating fellow migrants.

"What we're looking for is for an opportunity to improve our lives but also perhaps improve the health care system over there," she said. "I really like helping people who don't have easy access to healthcare."

U.S. officials worry that Trump's election will end the months-long lull in illegal border crossings, which plunged this year following an aggressive effort by Mexican officials to interdict migrants and President Biden's move in June to make most of those crossing into the country illegally ineligible for asylum. Larger numbers of migrants, officials have said, could be incentivized to cross into the U.S. unlawfully in the coming weeks, before Trump takes office on Jan. 20.

Alba Jaramillo, a Tucson-based immigration attorney, said Trump's win and the potential end to the CBP One system, could prompt more migrants to cross the southern border without authorization, including along dangerous parts of the Arizona desert where some perish trying to make it into the U.S.

"They're desperate," said Jaramillo, the co-executive director of the Immigration Law and Justice Network, a pro-immigrant organization. "I mean they have given up everything to come to the north."

Anjali Patil contributed reporting.