President Obama’s action to shield millions of undocumented immigrants from deportation and grant them work permits opens a new front in the decades-long debate over the scope of presidential authority.

Although Mr. Obama is not breaking new ground by using executive powers to carve out a quasi-legal status for certain categories of unauthorized immigrants — the Republican Presidents Dwight D. Eisenhower, Ronald Reagan and George H. W. Bush all did so — his decision will affect as many as five million immigrants, far more than the actions of those presidents.

Mr. Obama’s action is also a far more extensive reshaping of the nation’s immigration system.

“The magnitude and the formality of it is arguably unprecedented,” said Peter J. Spiro, a Temple University law professor. “It’s fair to say that we have never seen anything quite like this before in terms of the scale.”

The breadth of Mr. Obama’s decision is already raising serious legal and constitutional questions, fueling Republican charges of imperial overreach and worries among some Democrats of future fallout.