Sometimes two campaign promises can conflict. This happened after the Trump administration poured more resources into prosecuting migrants for crossing the Southwest border earlier in the year. What followed was a significant slowdown in the pursuit of drug traffickers, another key administration priority.

The decline turned around after Trump aides dispatched more lawyers to the border.

But the drop in drug prosecutions was eye-popping. Such cases declined by 24 percent in the six months after the Justice Department imposed a “zero tolerance” policy on migrants in early April, compared to the same period last year.

While the number of cases along the border has been generally decreasing in recent years, the decline accelerated immediately after the new policy, and by June had led to the fewest prosecutions in two decades.

Criminal charges for immigration violations — mostly illegal border crossing, a misdemeanor on the first offense — soared between April and September of this year to 60,684 in the five federal judicial districts on the border from Texas to California, a 121 percent increase from the same period a year earlier.