For years, migrants seeking asylum only needed to present themselves at a port of entry into the United States to begin the process. But the Trump administration has begun to use a system known as “metering,” which limits the daily number of asylum seekers allowed to present their cases at certain ports of entry.

The practice has given rise to informal waiting lists managed by the Mexican authorities, or even by the migrants themselves. There are currently more than 3,300 asylum seekers on the list in Ciudad Juárez. The number that can formally cross each day ranges from several dozen to none, and the wait can stretch for months. On Sunday only 10 were allowed to cross. The day before, zero.

In a trend that began during the Obama administration, an increasing percentage of migrants are traveling in family units with children. Some say they bring their children in part because they know they will have a better chance of being released from American detention more quickly.

“We heard in Guatemala that if you were traveling with children, it was much easier to cross,” said a woman, 30, who has been staying with her 5-year-old son at a Juárez shelter waiting for her turn to apply for asylum in neighboring El Paso. She gave her surname as Ramirez, but asked that her full name not be published because she did not want to imperil her case. She said she was fleeing a violent gang in Guatemala.

While President López Obrador has chosen, in some ways, to help the United States carry out its immigration agenda, he has also sent a message to potential migrants that they are now more welcome than ever in Mexico.

In the first three months of the López Obrador administration, there were 44 percent fewer deportations than during the last three months of President Enrique Peña Nieto’s administration.