In an address after the Orlando massacre punctuated with dire warnings of impending violence, Donald J. Trump said he would “suspend immigration from areas of the world when there is a proven history of terrorism” against the United States or its allies. Mr. Trump promised fixes to the immigration system that would be “tough” and “smart” and “fast.”

It sounded much like his provocative proposal to keep Muslims from entering the country, but those listening closely noticed an important change. By proposing to bar people from certain regions rather than religions, Mr. Trump had avoided the sticky issue of testing someone’s faith.

Mr. Trump’s plan, lawyers and legal scholars agree, is one that the president has the power to carry out.

But they said that putting it in place would take an ambitious bureaucratic effort not likely to move nearly as quickly as the candidate envisions. And it would make sweeping use of executive authority to enact the sharpest restrictions on immigration since 1965, when the United States abandoned longstanding quotas designed to exclude people from much of Asia and from southern and Eastern Europe.