Nogales, Arizona — The lull in illegal crossings at the U.S.-Mexico border continued in October, according to preliminary Customs and Border Protection figures obtained by CBS News. But officials worry that could be upended by the presidential election on Tuesday. 

Border Patrol agents recorded nearly 57,000 apprehensions of migrants between legal entry points along the U.S. southern border in October, internal federal statistics show. That’s slightly up from 54,000 in September, and very similar to the 58,000 and 56,000 apprehensions tallied in August and July, respectively.

The last time monthly apprehensions were lower than the levels seen in the past four months was four years ago, in September 2020, when Border Patrol apprehended fewer than 55,000 migrants along the U.S.-Mexico border, according to historical CBP data.

CBP’s figures don’t include those processed at legal border entry points, where the Biden administration has been admitting more than 1,000 migrants daily through an appointment system powered by a U.S. government phone app known as CBP One.

After spiking to a record 250,000 in December 2023, illegal crossings at the southern border dropped earlier this year, mainly due to aggressive efforts by Mexico to interdict U.S.-bound migrants. They plunged even further after President Biden invoked sweeping presidential powers to sharply limit asylum in early June, falling precipitously that month and at the beginning of July. Since then, migrant crossings have plateaued. 

While unlawful crossings remain low, some U.S. officials are concerned the presidential election could disturb the fragile equilibrium achieved at the southern border in recent months. Three U.S. officials said there could be a spike in illegal crossings if former President Donald Trump wins the election, as migrants try to enter the U.S. before he takes office in January.

“I could definitely see an increase, either a surge before the inauguration or a sustained increase after the election,” one of the U.S. officials said, requesting anonymity because they were not authorized to talk to the press.

While Vice President Kamala Harris has vowed to keep and strengthen Mr. Biden’s asylum restrictions, Trump has promised to seal the southern border altogether, including by discontinuing the CBP One app and other programs that allow migrants to enter the U.S. legally. He also pledged to reinstate his hardline immigration policies, like the Remain-in-Mexico program, and launch the largest deportation operation in American history.

A CBP official said the agency “remains vigilant to constantly shifting migration patterns — including the operations of transnational criminal organizations and other bad actors that facilitate human smuggling — and adjusts operations accordingly.”  

During its first three years in office, the Biden administration struggled with an unprecedented migration crisis at the southern border, fueled in part by record arrivals of migrants from crisis-stricken countries like Venezuela, where the U.S. cannot send deportees. It created scenes of chaos and coincided with mounting concerns among the public about border security. 

It also ushered a dramatic policy pivot by the Biden administration, which came into office promising to “rebuild” the U.S. asylum system. But the asylum crackdown Mr. Biden enacted in June relies on the same authority the Trump administration used to limit legal and illegal immigration, and disqualifies most migrants from U.S. protection. Those who use the CBP One app are exempted.

Adam Isacson, a migration policy analyst at the Washington Office on Latin America, a research and advocacy group, said he expects migration flows to remain relatively the same if Harris wins, as she has pledged to largely continue Biden’s policies. But he predicted a “big jump” in border crossings if Trump is elected, especially by those waiting in Mexico for a CBP One appointment.

“The message, both from smugglers and the messages from migrants to each other, will be ‘get there now. There’s a hard deadline,'” Isacson said, referring to Inauguration Day on Jan. 20, 2025.

Camilo Montoya-Galvez

Camilo Montoya-Galvez is the immigration reporter at CBS News. Based in Washington, he covers immigration policy and politics.