WASHINGTON (AP) — Arrests for illegal crossings on the U.S. border with Mexico fell by half in January from record highs in December to the third lowest month of Joe Biden’s presidency, authorities said Tuesday.

The sharp drop is welcome news for the White House, even if it proves temporary, as immigration becomes one of the biggest issues in this year’s presidential election, with exit polls showing it is the top concern among many Republican voters in early primaries. House Republicans are waging a campaign to impeach U.S. Homeland Security Secretary Alejandro Mayorkas for his oversight of the border.

Seasonal declines and heightened enforcement by the U.S. and its allies led to the sharp decline, said Troy Miller, acting commissioner of U.S. Customs and Border Protection. U.S. authorities have repeatedly praised Mexico for a crackdown launched in late December.

Border Patrol arrests totaled 124,220 in January, down 50% from 249,735 in December, the highest monthly tally on record. Arrests of Venezuelans plunged 91% to 4,422 from 46,920 in December.

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Numbers ebb and flow, and the January decline may prove tenuous. Panama reported that 36,001 migrants traversed the notorious Darien Gap in January, up 46% from December. The vast majority who cross the Panamanian jungle are Venezuelans headed to the United States, with considerable numbers from Haiti, China, Ecuador and Colombia.

“We continue to experience serious challenges along our border which surpass the capacity of the immigration system,” Miller said.

Tucson, Arizona, was again the busiest sector for illegal crossings with 50,565 arrests, down 37% from December, followed by San Diego. Arrests in the Border Patrol’s Del Rio sector, which includes the city of Eagle Pass, the main focus of Texas Gov. Greg Abbott’s border enforcement efforts, plummeted 76% from December to 16,712, the lowest since December 2021. Arrests in Texas’ Rio Grande Valley, dropped 60% to 7,340, the lowest since July 2020.

The only months of Biden’s presidency with fewer border arrests were in June 2023, after pandemic-related asylum restrictions lifted, and February 2021, his first full month in office.

During an interview with The Associated Press in January, Mayorkas said that Mexico’s immigration enforcement agency didn’t have the funds in December to carry out enforcement actions but when that was rectified, there was an immediate and substantial drop in the number of migrants encountered at the southern border.

When including migrants who were allowed to enter the United States under new or expanded legal pathways, migrant encounters totaled 176,205 in January after topping 300,000 for the first time in December. U.S. authorities admitted about 45,000 people at land crossings with Mexico in January through an online appointment system called CBP One, bringing the total to 459,118 since it was introduced a year earlier.