Three consecutive U.S. administrations have turned to Mexico for help with immigration enforcement at moments of crisis along the U.S. southern border, and when Vice President Harris meets virtually with Mexican leader Andrés Manuel López Obrador on Friday, the United States will once more arrive in need of a favor.

Since President Biden took office, the number of migrants taken into U.S. custody along the border has soared to the highest levels in nearly 20 years, surpassing 172,000 in March. His administration has opened more than a dozen emergency shelters to care for record numbers of teenagers and children arriving without parents. Biden’s handling of the migration influx at the border ranks among his worst-polling issues, and he has tasked Harris — his party’s heir apparent — with leading an international effort to address the root causes of migration and stem the flow.

Mexico is central to that plan, underscoring what has become a growing U.S. dependence on Mexico to carry out immigration enforcement functions at a time when such measures are subject to frequent legal challenges in U.S. courts or politically unpalatable to Democrats.

Mexico’s ability to limit migration has given its government significant leverage over an issue that is a political vulnerability for Biden. It also lands the Biden administration in the awkward position of asking Mexico to intensify its enforcement efforts after easing U.S. border controls by rolling back Trump-era policies, including deals made with López Obrador.

In March, as Biden officials scrambled to address the surge, the Mexican government agreed to deploy thousands of troops and police officers to stop migrants, while declaring its southern border closed for nonessential travel as a public health measure for the first time since the start of the pandemic. As part of those talks, the United States agreed to send millions of surplus AstraZeneca vaccine doses to Mexico, and López Obrador is seeking additional supplies.

Juan Gonzalez, Biden’s top Latin America adviser on the National Security Council, said the president isn’t narrowly focused on the U.S.-Mexico border and is partnering with Mexico to develop a comprehensive approach to migration in the region.

“We need Mexico, and Mexico needs us,” Gonzalez said in an interview. “This president’s approach was to engage early and constructively, rather than to threaten, because he recognized that Mexico’s interests were aligned with ours on migration.”

“To date, they’ve increased their enforcement, deploying 10,000 [security personnel] to their southern border and have expanded their asylum capability,” Gonzalez added. “But the conversation with Mexico is about more than enforcement on their southern border. Yes, we talk about enforcement, but also about support for humanitarian programs, addressing the root causes of migration and promoting economic investment.”

Mexico’s pivotal role in U.S. plans for a wider, regional migration management strategy amounts to a remarkable evolution from the 1980s, when Mexico was primarily a sender of migrants. Later it became a transit country for Central Americans as well as a destination for those seeking employment in Mexican factories.

More recently, Mexico has begun to resemble an “interdiction state,” acting as a buffer zone where enforcement activities are fluid and subject to geopolitical negotiation, not unlike the role Turkey has played with the European Union, according to Cris Ramón, an immigration policy analyst in Washington.

“Turkey is an interdiction state because it holds refugees,” Ramón said. “In Mexico, the U.S. approach creates an interdiction state by seeking more enforcement. In both cases you’re externalizing your borders and making another nation your border authority.”

Added Ramón: “At a certain point you can’t keep relying on another country without that country recognizing it’s central to your regional strategy, and they will try to leverage that.”

That leverage can take different forms, but it could come in handy for López Obrador at a time when the Biden administration is pushing a regional anti-corruption strategy and prodding Mexico on a number of other initiatives, from climate change commitments and clean energy to labor protections. Though López Obrador campaigned as an anti-corruption crusader, his party includes more dubious figures who lack his reputation for personal rectitude.

López Obrador’s approval ratings remain high, and his party is vying for a legislative supermajority in upcoming elections next month that could bolster his ambitions to transform Mexico. His critics decry what they see as authoritarian tendencies and a nationalist worldview that jeopardizes the close U.S.-Mexico relations and economic ties cultivated by his predecessors.

Luis Rubio, president of the think tank México Evalúa, said that López Obrador intended to use that leverage to keep the U.S. government from interfering in internal Mexican affairs.

“I don’t doubt that the armed forces were removed from our southern border to gain leverage in the relationship,” said Rubio, referring to the drawdown in troops along Mexico’s border with Guatemala over the past year. “But the fundamental objective [of the Mexican government] is to reduce the Biden administration’s interest in getting involved in internal issues” such as Mexican elections and corruption, he said.

The Biden administration announced soon after taking office that it would not use a Trump-era public health order to send home unaccompanied Central American teenagers and children, allowing them to pursue humanitarian claims under U.S. law. In March, more than 18,800 minors arrived without parents, the highest total ever, and the Biden administration is now holding more than 22,500 teens and children in shelters.

The number of parents arriving with children also skyrocketed in February and March, and most of those families were released into the U.S. interior, because Mexico said it could not take them back, according to U.S. officials. The Biden administration has asked Mexico to accept more of those families, who migrant advocates say are vulnerable to kidnapping and extortion once they are returned to Mexican border cities.

Mexican officials including Foreign Minister Marcelo Ebrard have been privately frustrated with Biden’s rapid-fire rollback of Trump policies, according to current and former U.S. and Mexican officials, because Mexico thought Biden’s moves incentivized migration in the short term while his proposed solutions consisted of long-term measures that could take years to make a difference.

The Mexican government arrests and deports minors and families with varying levels of intensity, often describing its actions as “rescues” that save Central Americans from criminals, even though in many cases the migrants themselves have hired the smugglers.

The latest statistics from Mexico’s National Migration Institute show a 49 percent increase in immigration arrests from January to March, and a 72 percent jump in deportations during that same period. But Mexico’s enforcement capacity is limited, and deportations in March totaled 7,946, fewer than the average number of migrants U.S. authorities take into custody over the course of two days.

U.S. border officials say increased Mexican enforcement in March, however modest, has helped prevent the current migration wave from growing larger. Preliminary data from U.S. Customs and Border Protection indicates the number of families and unaccompanied minors taken into U.S. custody declined slightly from March to April, a period when U.S. officials were projecting an increase.

When President Barack Obama faced the first major influx of Central American minors and families in 2014, his administration leaned on Mexico to crack down along the freight rail lines many Central Americans were using to travel north. Mexico announced a “Plan Frontera Sur” to harden its southern border with Guatemala, where migrants and illicit goods have long flowed freely through unguarded, informal crossings.

President Donald Trump turned to Mexico, too, especially in 2019 amid a record influx of Central American parents arriving with children. As groups of migrants numbering 300 or more crossed into the United States, Trump fulminated and threatened to sink Mexico’s economy with escalating tariffs.

López Obrador responded with an unprecedented militarized crackdown along Mexico’s northern and southern borders, while agreeing to a broad expansion of Trump’s “Remain in Mexico” program that required asylum seekers to wait outside U.S. territory while their claims were processed in U.S. courts. Mexico deported more than 24,000 in June 2019 during the first month of the troop deployment — three times the number it deported last month.

Migration levels fell sharply after the crackdown, but the Mexican military deployment waned over the final months of Trump’s presidency, coinciding with a period during which U.S. agents also began detaining more Mexican adults crossing the border as the country’s economy reeled from the pandemic.

López Obrador ran for president in 2018 as a leftist, telling crowds Mexico would no longer do the Americans’ “dirty work” by intercepting migrants headed north. He continued with a welcoming message once in office, but that changed as large caravans of Central American migrants began streaming through, piquing Trump’s anger.

Trump’s threats coerced López Obrador into a reversal, but also had the effect of intermingling immigration enforcement with other bilateral issues beyond trade, including corruption, environmental protection and counternarcotics cooperation.

Rubio noted that before Trump, U.S. administrations had committed to compartmentalizing issues like migration, drug trafficking and commerce — so a dispute in one area wouldn’t contaminate the entire relationship. That arrangement was scuttled by Trump.

“Trump violated one of the basic premises of the relationship” that dated back to the 1980s, he said.

“I think all the leverage he will construct is for this,” Rubio said. “I have no doubt he’ll use it.”

Christopher Landau, who was U.S. ambassador to Mexico under Trump, said the increased attention on migration in the bilateral relationship “can cut both ways.”

“It was seen as very bad for Mexico when Trump brought up migration, but it’s something the Mexicans can use to get more leverage of their own,” he said in an interview.

“Mexico is enforcing its own migration laws, but to the extent we are urging Mexico to undertake enforcement actions, one would expect us to look more kindly on any request Mexico will have of us.”

In a video conference last week cited by Mexico’s Reforma newspaper, Landau said López Obrador knew that the migration issue was important to Trump and that as long as Mexico cooperated, U.S. officials “weren’t giving him a hard time on a gazillion other issues.”

López Obrador has made clear he understands the issue is also important to Biden. When Biden and his top aides were insisting in March that the border influx was part of a normal seasonal pattern, López Obrador told reporters that migrants have been encouraged by the U.S. president’s more welcoming tone.

“Expectations were created that with the government of President Biden there would be better treatment of migrants,” López Obrador said. “And this has caused Central American migrants, and also from our country, wanting to cross the border thinking that it is easier to do so.”

One potential problem for López Obrador in talks with Harris: the soaring number of Mexican adult migrants coming across the U.S. border — now at levels that resemble the early 2000s.

“The rising number of Mexicans who are migrating because of a lack of economic opportunity suggests that’s not going to end soon,” said Duncan Wood, a Mexico expert at the Wilson Center in Washington. Any measures that would attempt to stop Mexicans from heading north “would be disastrous” for López Obrador, he said.