President Obama’s executive action to protect millions of unauthorized immigrants from deportation is an act that both follows and departs from precedents set by his predecessors.

As immigrant advocates — and the White House itself — point out, presidents have a long history of using their discretionary enforcement powers to allow people to enter and remain in the country outside the regular immigration laws. But Obama’s move offers relief to more people than any other executive action in recent history — about 3.9 million people, or roughly 35% of the estimated total unauthorized-immigrant population — a point that some opponents have used to differentiate Obama’s action from those of past presidents.

Obama’s announcement follows his decision in June 2012 to grant temporary reprieves from deportation for 1.5 million eligible unauthorized immigrants who’d been brought to the U.S. as children — the program known as Deferred Action for Childhood Arrivals, or DACA. In the memorandum announcing DACA, then-Homeland Security Secretary Janet Napolitano framed it as part of the executive branch’s role “to set forth policy for the exercise of discretion within the framework of the existing law.” Obama’s action expands that program, and protects other groups, using a similar rationale.

Most previous executive actions on immigration were targeted fairly narrowly, according to a summary compiled by the American Immigration Council. The 39 “executive grants of temporary immigration relief” since 1956 listed by the council covered, among other groups, Ethiopians fleeing that country’s Marxist military dictatorship in the 1970s, Liberians who escaped their country’s brutal civil wars, and foreign students whose academic eligibility was interrupted by Hurricane Katrina.

Other actions taken by prior administrations affected considerably more people. Most of them were eventually formalized or superseded by legislation, though sometimes — as often happens with complicated subjects such as immigration — the new laws led to new issues...