One of the main reasons Vice President Kamala Harris lost the election is the Biden administration’s record on immigration and the border — polls show it ranks up close with inflation among the top issues that drove swing voters to Trump.

And the recriminations about how Democrats got so out of step with the public on this issue are well underway. Part of the story, as The Atlantic’s Rogé Karma and others have written, involves a badly wrong electoral theory that held that support of unauthorized immigrants was key to winning over Latino voters. And part of it is the increased influence of progressive advocacy groups who pushed the party left.

But the true heart of Democrats’ political disaster on immigration is a policy failure from top Biden officials.

These officials — including the president — understood perfectly well that a border surge was politically perilous. They sought to drive down arrivals, starting early in Biden’s first year in office: All the way back in March 2021, the New York Times reported, Biden was furious about the border crisis, demanding to know whom he needed to fire to fix it.

The problem was that, for three full years, Biden’s team proved unable to fix it.

Until, in 2024, they suddenly did.

During Biden’s first three years in office, the number of arriving migrants skyrocketed, leading to a backlash as even blue states and cities complained they were overwhelmed. The peak came in December 2023, a month when officials reported about 250,000 encounters with migrants at the border, a record.

Then, starting early in 2024, and continuing throughout the year, border arrivals plummeted. In August, border encounters had dropped to about 58,000 — 77 percent lower than the previous December’s level. By the end of the year, they’d dropped even further.

Some Democrats touted this as a policy success for Biden — he himself bragged about it at his debate with Trump. But the timeline raises some questions.

For instance: If it was possible all along to get the border much more under control, why didn’t Biden do it years ago? If this was what Biden hoped to achieve all along, what went wrong in the administration’s decision-making that it only materialized after years of political pain?

The answers are central to reckoning with Biden’s border record, and with how Democrats should handle the issue in the future.

What sent border arrival numbers plummeting in 2024?

Typically, retrospectives like these begin chronologically, at the start of the administration. But in this case, it’s more helpful to start near the end — with the question of why border arrivals actually did plummet in 2024.

Progressives have often tended to argue that surges in unauthorized immigration are fundamentally outside the president’s control — that, overall, migration trends are driven by broader structural factors, like poor governance and economic conditions in the migrants’ home country, and the relative strength of the US economy.

Yet the Latin American countries from which people have been fleeing did not suddenly grow far more stable in 2024, and economic conditions in the US were pretty similar to 2023. So this doesn’t satisfactorily explain the dramatic change in the past year.

Rather, there were two important policy changes that occurred that year: one south of the border, and one north of it.

First, the Biden administration got........

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