Last week, President-elect Donald Trump re-emphasized the approach he will take in enforcing the nation's immigration laws, which is much different from the manner of enforcement utilized by President Barack Obama. The latter pointedly declined to deport the 5 million undocumented immigrants in the United States who are the parents of children born here — children who, by virtue of birth, are American citizens. Trump has made known his intention to deport all undocumented people, irrespective of family relationships, starting with those who have committed crimes.

In response to Trump's stated intentions, many cities — including New York, Chicago, Los Angeles and San Francisco — have offered sanctuary to those whose presence has been jeopardized by the president-elect's plan. Can they do this?

Here is the back story.

Under the Constitution, the president is the chief federal law enforcement officer in the land. Though the president's job is to enforce all federal laws, as a practical matter, the federal government lacks the resources to do that. As well, the president is vested with what is known as prosecutorial discretion. That enables him to place priority on the enforcement of certain federal laws and put the enforcement of others on the back burner.

Over time — and with more than 4,000 criminal laws in the United States Code — Congress and the courts have simply deferred to the president and permitted him to enforce what he wants and not enforce what he doesn't want. Until now.

Earlier this year, two federal courts enjoined President Obama — and the Supreme Court, in a tie vote, declined to interfere with those injunctions — from establishing a formal program whereby undocumented people who are the parents of natural-born citizens may lawfully remain here. It is one thing, the courts ruled, for the president to prioritize federal law enforcement; it is quite another for him to attempt to rewrite the laws and put them at odds with what Congress has written. It is one thing for the president, for humanitarian reasons or because of a lack of resources, to look the other way in the face of unenforced federal law. It is another for him to claim that by doing so, he may constitutionally change federal law.

Trump brilliantly seized upon this — and the electorate's general below-the-radar-screen disenchantment with it — during his successful presidential campaign by promising to deport all 13 million undocumented immigrants currently in the United States, though he later reduced that promise so as to cover only the 2 million among them who have been convicted in the United States of violating state or federal laws.