(Bloomberg Opinion) -- Since 2017, more than one million Central Americans have made their way to the U.S. southwestern border, triggering a disjointed but brutal crackdown by the administration of President Donald Trump. Although the combination of tighter border controls and the coronavirus has reduced these flows, they will resume when the Covid-19 lockdowns lift.

Only this time, Mexicans are likely to join the exodus. The resulting tensions could destabilize one of the worlds most tightly woven bilateral relationships, jeopardizing cooperation on everything from counternarcotics to water rights and the prosperity that closer ties have underpinned on both sides of the border.

Mexican migration to the U.S. peaked at the turn of the last century. At the end of the 1990s and early 2000s, hundreds of thousands of Mexicans moved north every year, many evading border sentries along the way. They fanned out across the nation, drawn to enclaves in California, Texas, Illinois, and Arizona, but also to newer locations: Colorado, Florida, Georgia and Idaho. And many switched from seasonal work in the fields to more permanent year-round jobs in childcare, landscaping, hotels and car services.

By the mid-2000s, the exodus slowed. For the past 15 years, more Mexicans have left the U.S. than come each year. This shift reflects economic progress at home, not least an end to the financial booms and busts of the 1980s and 1990s. Beefed-up enforcement at the U.S. border has also discouraged circular migration, with workers now rarely returning home for a few months between planting seasons.

Better schooling also helped. With the number of years of education nearly doubling since 1990, the average Mexican 16-year-old is in class, not the workforce. So have changing demographics: Starting in the 1980s Mexican families have had fewer kids, now averaging just over two per household. Compared with the 1990s, fewer Mexicans are turning 18 every year and searching for work either at home or in the U.S.

But in place of Mexicans came a swelling wave of Central Americans, driven by poverty, violence and devastating droughts due to climate change. The majority have been women and children, pulled, too, by the presence of family, friends and economic ties in the U.S.

The Trump administration has made aggressive efforts to stop them. It changed asylum rules, attempting to disqualify those fleeing gang or domestic violence, to limit the right to apply to those arriving at official border crossings, and to otherwise make it more difficult to seek protection. Those families who did enter the U.S. system were often subjected to inhumane living conditions, with children separated from parents and placed in detention pens resembling cages.

The U.S. leaned hard on Central American governments to stop these would-be migrants from leaving in the first place. Under pressure, Mexico also acquiesced to holding tens of thousands of Central Americans for months or more as they waited to have their claims heard in U.S. immigration courts.

The number of Central American migrants did decline. In the start of 2020, flows fell almost by half compared with the year before. With Covid-19 restrictions, the movement nearly ceased in April and May. Yet the reasons pushing families to leave havent changed. Instead, the pandemic is making them all the worse. And not just in Central America, but also in Mexico.

The biggest factor driving a resurgence of Mexicans north is economic desperation: Mexicos economy is expected to shrink by more than 10% this year. Even before the pandemic, both public and private investment had fallen to historic lows. Since then more than 12 million Mexicans have lost their livelihoods, as the government is doing little to keep companies going or preserve jobs. And in addition to the consequences of President Andres Manuel Lopez Obradors misguided economic policies, his reversal of education reforms has made it less important and likely that students will stay in school. Those who do will be less likely to learn the skills needed in a 21st-century Mexican economy.

Rising violence is also driving hundreds of thousands of Mexicans from their homes and communities. Last year homicides topped 34,000. The first half of 2020 has been even more deadly.

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As these factors push Mexicans to leave, economic and familial ties pull them north. Mexicans represent the biggest migrant population in the U.S. (the majority here legally). Even with a soft U.S. economy, these fellow citizens can provide a contact, a first place to stay, and a lead on a job for future aspiring migrants.

If the past is any guide, many more Mexicans will head north. Their numbers are already ticking up: Since January, more Mexicans than Central Americans have been apprehended at the border.

The Trump administrations methods to discourage Central Americans wont work with Mexico. Lopez Obrador and his National Guard arent able to stop citizens who have a constitutional right to leave their country. Mexican migrants are less likely to be asylum seekers (even as many flee incredible violence), so the rule changes wont dissuade their journeys. And Mexicans are also more likely to succeed in making it into the U.S.; the nations proximity means that those who have been deported can easily try their luck again.

A migration surge could be a game changer for U.S. politics and policy. On the foreign policy side, it could rupture the bonhomie between Lopez Obrador and Trump, as migration becomes a defining electoral campaign issue. Mexicos president has so far ignored or endured U.S. slights, but a full frontal attack on his citizens would be harder to take given his long-standing (and popular) defense of Mexican migrants.

For the U.S. presidential race, a surge in Mexican migration would mobilize both sides. It would provide anti-immigrant fodder that Trump could use to feed his base. But his tirades could also motivate more of the tens of millions of Mexican-Americans, weary of the ugliness directed at them by association, to turn out to vote. With Latinos representing 13% of the electorate, Democratscould benefit.

The hardest part will come later. Whoever wins in November wont have the policy tools to manage this migration effectively or humanely. Outdated laws and an already strained immigration system provide little recourse, and political polarization makes it all the harder to fix them. Mexican migration could easily become the new administrations first big crisis.

This column does not necessarily reflect the opinion of the editorial board or Bloomberg LP and its owners.

Shannon O'Neil is a senior fellow for Latin America Studies at the Council on Foreign Relations in New York.

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