The San Diego region, like many communities that hug the southern border, saw a sharp drop in migrants entering the United States after the Biden administration made it harder to apply for asylum.

But thousands of newcomers who had crossed the border haven’t been forgotten, and even as many of them made their way to other corners of the country, some remain in and around San Diego, still undocumented.

Now, with President-elect Donald J. Trump pledging to carry out mass deportations, leaders in San Diego have been weighing how far to go in trying to protect people vulnerable to deportation and how much local law enforcement should cooperate with federal agents.

Such discussions are unfolding around the country after Mr. Trump won back the White House promising to curb immigration and tighten up border security.

But few places have been seeing the debate play out as dramatically as San Diego County, which sits on California’s border with Mexico, and where for a few weeks this year, the number of crossings was higher than in Texas and Arizona.

After Mr. Trump’s victory, the Board of Supervisors for San Diego County moved to bolster protections for migrants by requiring federal agents to obtain a judicial warrant for any undocumented immigrants they want to pick up from a local jail, banning any investigative interviews by immigration officials inside jails and prohibiting the use of county resources for immigration enforcement.