Under the Biden administration, they say, Customs and Border Patrol agents now deposit migrants at some of the most obscure, understaffed checkpoints, leaving their Mexican counterparts scrambling when they discover dozens of migrants walking in from the United States.

Local government officials in Ciudad Juárez and shelter operators say Mexico is dialing up operations to capture and deport migrants along the northern border. On a near daily basis, two of them said, Mexican authorities are stopping vans stuffed with families and pickup trucks carrying livestock — along with migrants crouching on the floor to avoid detection.

Part of the reason Mexico is willing to continue cracking down is that, despite being a country that has long sent people north, there is a lot of resentment toward Central American migrants.

“The level of negative attitudes that we have toward migrant flows has gone up, so there won’t be a political cost” for Mr. López Obrador, said Tonatiuh Guillén, who ran Mexico’s National Migration Institute in the first half of 2019. “But with Trump, we negotiated nothing — we gave them a lot and they didn’t give us anything back,” he added, arguing that the strategy should be different with Mr. Biden.

Despite the very public tensions with Mexico under Mr. Trump, Mr. López Obrador has been wary of the Biden administration, concerned that it might be more willing to interfere on domestic issues like labor rights or the environment.

Instead, several Mexican officials say, his government has pushed the United States to deter Central Americans from migrating by sending humanitarian aid to Honduras and Guatemala in the wake of two hurricanes that devastated those countries and, many experts believe, pushed even more people to migrate.

Mexican officials have also asked the United States to send more Hondurans and Guatemalans apprehended in the United States directly to their home countries, rather than releasing them to Mexico, making it even harder for them to try to cross the border again.

While the negotiations over migration may be on a separate track from Mexico’s request for surplus vaccines from the United States, the need for them in Mexico is clear.

About 200,000 people have died in Mexico from the virus — the third highest death toll in the world — and the country has been relatively slow to vaccinate its population. That poses a potential political risk for Mr. López Obrador, whose party is heading into crucial elections in June that will determine whether the president hangs onto control of the legislature.

“Mexico needs cooperation from the U.S. in getting its economy jump-started and getting vaccines to get out of the health crisis,” said Andrew Selee, president of the Migration Policy Institute in Washington. “So there’s room for the two countries to reach agreements based on aligned interests rather than overt threats.”

Zolan Kanno-Youngs and Michael D. Shear contributed reporting from Washington, D.C.

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