Washington — U.S. immigration officials along the southern border are on track to process more than 300,000 migrants in December, an all-time monthly high that will likely include record numbers of families traveling with children, according to internal government data obtained by CBS News.

The extraordinary number of migrant arrivals this month is the most dire juncture yet of a three-year-long crisis at the U.S.-Mexico border that has strained resources in small and large U.S. communities, left countless of migrants in limbo, prompted lawmakers to consider drastically limiting asylum and created a major political vulnerability for President Biden as he seeks reelection.

U.S. Border Patrol agents at the Mexican border are on course to take into custody this month a quarter of a million migrants who entered the country illegally, while their colleagues at official ports of entry are expected to process roughly 50,000 new arrivals under a Biden administration appointment system. 

Never before has U.S. Customs and Border Protection processed that many migrants along the southern border in one month, according to monthly tallies going back to fiscal year 2000. The previous monthly high in overall crossings at the southern border was recorded in September, when the agency processed nearly 270,000 migrants at and in between ports of entry.

In the first 28 days of December, Border Patrol agents processed nearly 235,000 migrants who crossed the southern border illegally in between ports of entry, averaging roughly 8,400 apprehensions each day, the preliminary Department of Homeland Security (DHS) statistics show. If the pace continues, Border Patrol will report roughly 260,000 apprehensions this month, a record high.

Through Dec. 28, Border Patrol had processed nearly 96,000 migrant parents and children traveling together, the data show, putting it on track to match or surpass the 103,000 family member apprehensions it recorded in September, the current monthly high for this demographic. Families pose a unique challenge for federal officials given the vulnerability of children and legal and operational limits on the detention of minors. 

Border Patrol also processed 127,000 single adult migrants and nearly 12,000 unaccompanied children during December's first 28 days, according to the DHS statistics. 

The spike in illegal crossings in December was partially driven by Venezuelan arrivals, which had dropped in the fall after the Biden administration announced it would conduct direct deportations to crisis-stricken Venezuela. Border Patrol has processed 50,000 Venezuelan migrants who entered the country illegally in December, compared to 23,000 last month, the internal data show.

The internal statistics do not account for the last three days in December. CBS News first reported Thursday that illegal border crossings alone had reached a monthly record last month.

The biggest test yet for Biden's border policies

The record spike in migration this month is the biggest test yet for the Biden administration's border strategy, which has sought to reduce illegal crossings by diverting migrants to programs that allow them to enter the U.S. legally and enhance penalties for those who bypass those channels.

The Biden administration has been allowing migrants in Mexico to secure one of 1,450 appointments each day though a smartphone app, so they can be processed by U.S. officials in an orderly way at ports of entry. Another program is allowing up to 30,000 migrants from Cuba, Haiti, Nicaragua and Venezuela to fly to American airports if U.S.-based individuals agree to be their financial sponsors.

Officials paired those programs with a regulation that makes it harder for migrants to obtain asylum if they fail to use these legal channels and cross into the U.S. illegally without first seeking protection in another country. 

While the administration credited this approach for a two-year low in migrant apprehensions in June, the strategy has faltered in recent months, as the immigration system buckles under the weight of unprecedented migration flows.

Officials, for example, do not have the manpower and resources to screen and detain all migrants who could be subjected to the asylum restriction. In fact, government figures indicate that most migrants who entered the U.S. illegally in recent months have been released with court notices, without any asylum screenings.

On average, those court cases take years to complete due a massive backlog of claims. The immigration court system, staffed by fewer than 800 immigration judges, is overseeing 3 million pending cases.

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