Jean Guerrero | KPBS

Fifty-year-old Teresa Gomez moved to the United States as a young woman with one goal: to support her aging mother in Mexico.

For decades, Gomez has sent her mother a third of every paycheck. She organizes merchandise and attends customers at a Barrio Logan grocery store. Her mother has heart problems, and uses the money to survive in the southern Mexican state of Jalisco.

"I send her $400 every two weeks," said Gomez, who has dual U.S.-Mexico citizenship. "I provide for my mother."

But that money - known as remittances - may soon be in jeopardy. President Donald Trump has threatened to target remittances as a way of forcing Mexico to pay for his planned border wall, which will cost anywhere between $5-25 billion.

Gomez doesn't think that would be fair. She's a U.S. citizen as well as a Mexican one.

"How are you going to punish so many citizens who come as workers, with good thoughts and acts in this country?" Gomez asked.

Remittances to Mexico reached a record $27 billion last year, representing the largest source of foreign income for the country after auto exports. Immigrants in California sent the largest chunk, $8 billion.

Gomez, a single woman, said she wouldn't be able to afford a tax on remittances. She rents a small room in a Barrio Logan apartment, where she lives with her talkative conure parrot, Kiko.

"We came here with the American dream, not to be millionaires, but simply to live decently and help our families," she said.

Gomez is one of millions of Mexicans in the U.S. who send money to relatives south of the border. She sends the money through a small money-transfer shop, El Frijolito, which charges a fee of five or ten dollars, depending on the desired exchange rate. Others use larger transfer organizations such as Western Union.

The recent surge in remittances was due in part to fears about Trump's threats. The owner of El Frijolito, Mariceli Castro, said money transfers surged the day after Trump became president.

"The lines were all the way to the door, people were sending large quantities of money, a lot of people were selling everything they had in the bank," Castro said.

She said if Trump targets remittances, it would be devastating for her business and her clients.

"Most of (my customers) have an honest job, earning money with the sweat of their foreheads just so they can send it home," Castro said.

Targeting remittances may be trickier than Trump has outlined in his campaign memos. Policy experts have said that a tax on remittances would unnecessarily hurt all money transfers to Mexico, including those by Americans.

"It would have to be broadly applied - it would be, basically, a tax on electronic transfers," said Gordon Hanson, dean of UC San Diego's School of Global Policy and Strategy. "The thing is, what's the difference between an electronic transfer that is a remittance versus a transfer between two business entities? On paper, they look identical."

Hanson said a tax or ban on remittances would be devastating for Mexican-Americans and the San Diego economy in general, not just people who are in the U.S. illegally.

"You look at our construction industry, you look at bars and restaurants and hotels so the San Diego tourist industry, go into any San Diego neighborhood, who is caring for yards, who is cleaning houses and providing child care?" he said.

Gomez said she will find a way to send her mother the money she needs for medicines and food - no matter what.

"As a human being, or rather, as a Mexican, nothing is impossible," she said.

If need be, Gomez could cross the border to transfer the money in Tijuana. She would have to contend with three- to four-hour wait times at the ports of entry, but she said it would be worth it.

For immigrants who live far from the border, or who lack legal immigration status in the U.S., it's more complicated.

Marina is a 37-year-old immigrant who asked us to keep her identity secret for fear of deportation. She cleans houses and takes care of a two-year-old girl, sending about $500 a month to her mother in Mexico City. She is also single.

"I never go out - it's rare for me to go watch a movie or eat with a friend because I'm always working," she said.

She said she moved to the U.S. when she was 21 years old, in 1999, to support her mother, and that Trump's threats scare her because her mother depends on her. She said she is afraid of going back to Mexico, where homicides rose by 22 percent last year.

"When I talk to my family, they tell me every day it's worse. They're even afraid to go out into the street because they killed someone here, they killed someone there," she said.

She said if Trump wants to build a wall, he should build one - but she hopes he finds another way to pay for it, because she can't afford it.

"I respect the new president because this is simply not my country," she said. "But I'm not robbing money as some people say. I work."