Families at the border will present a familiar test for Biden’s Homeland Security pick.

Alejandro N. Mayorkas, President-elect Joseph R. Biden Jr.’s choice to lead the Department of Homeland Security, will immediately face the challenge of rolling back President Trump’s restrictive immigration policies, while balancing demands from the left for more lenient policies with those of the moderates who see any show of tolerance as an instigator to an uptick in illegal migration.

While Mr. Trump’s approach to legal and illegal immigration has been more extreme than that of several previous administrations, the balancing act is a challenge Mr. Mayorkas has faced before.

“Many have taken great issue with the administration’s removal of individuals who have not qualified for refugee status or asylum status in the United States and our practice of removing those who have not qualified for relief under law,” Mr. Mayorkas said during a 2016 address at Georgetown Law when he was the deputy secretary of the department under former President Barack Obama. “Whether we expand the basis of which we seek to welcome these individuals fleeing for a better life is a question that is answered by thinking of who we want to be as a country.”

Many of the Trump administration’s policies cannot be immediately undone, and Mr. Biden is likely to face an early test of human consequences, as indicators suggest that migration will swell at the southwestern border with Mr. Trump’s pending departure.

In November, border officials apprehended a child crossing the border alone 4,467 times. That is a slight drop from the 4,661 in October, but a stark increase from the 712 recorded in April, when various countries imposed national lockdowns and the Trump administration invoked a public health emergency rule to put new border restrictions in place.