Arrests for illegally crossing the border from Mexico fell 7% in September to a more than four-year low, authorities said Tuesday. It was likely the last monthly gauge during a presidential campaign in which Republican nominee Donald Trump has made immigration a signature issue.

The Border Patrol made 53,858 arrests, down from 58,009 in August and the lowest tally since August 2020, when arrests totaled 47,283, according to U.S. Customs and Border Protection.

Mexicans accounted for nearly half of the arrests, becoming a greater part of the mix. In December, when arrests reached an all-time high of 250,000, Mexicans made up fewer than 1 in 4. Arrests for other major nationalities seen at the border, including Guatemalans, Hondurans, Colombians and Ecuadoreans, have plunged this year.

San Diego was again the busiest corridor for illegal crossings in September, followed by El Paso, Texas, and Tucson, Arizona.

For the government's fiscal year that ended on September 30, the Border Patrol made 1.53 million arrests after topping 2 million in each of the previous two years for the first time.

The White House touted the numbers as proof that severe asylum restrictions introduced in June were having the intended effect, and blamed congressional Republicans for opposing a border security bill that failed in February. Vice President Kamala Harris has used that line of attack against Trump to try to blunt criticism that the Biden administration has been weak on immigration enforcement.

"The Biden Harris administration has taken effective action, and Republican officials continue to do nothing," said White House spokesman Angelo Fernández Hernández.

The Federation for American Immigration Reform, a frequent administration critic and advocate for immigration restrictions, attributed recent declines to more enforcement by Mexican officials within their own borders, saying the White House "essentially outsourced U.S. border security to Mexico in advance of the 2024 election — policies that can be reversed at any time that the government of Mexico chooses."

Arrests fell sharply after Mexico increased enforcement in December, and took a steeper dive after the U.S. asylum restrictions took effect in June. U.S. officials have not been shy about highlighting Mexico's role.

Mexican authorities are encountering more migrants this year while deportations remain relatively low, creating a bottleneck. Panamanian authorities reported an increase in migrants walking through the notorious Darien Gap during September, though numbers are still well below last year.

Troy Miller, acting CBP commissioner, said last week that the administration is working with Mexico and other countries to jointly address migration.

"We continue to be concerned about any bottlenecks, we continue to look at those, we continue to address them with our partners," Miller said at a news conference in San Diego.

The Biden administration has promoted new and expanded legal pathways to enter the country in an effort to discourage illegal crossings. In September, CBP allowed more than 44,600 people to enter with appointments on an online system called CBP One, bringing the total to 852,000 since it was introduced in January 2023.

Another Biden policy allows up to 30,000 people from Cuba, Haiti, Nicaragua and Venezuela with financial sponsors to enter monthly through airports. More than 531,000 people from those four countries have entered that way up through September.