None of the times emergency powers have been invoked since 1976, the year Congress enacted the National Emergencies Act, involved a president making an end run around lawmakers to spend money on a project they had decided against funding. Mr. Trump, by contrast, is challenging the bedrock principle that the legislative branch controls the government’s purse.

“On the surface, this ‘Oh, other presidents do this, too’ line seems logical,” said Chris Edelson, an American University government professor and author of a 2013 book, “Emergency Presidential Power: From the Drafting of the Constitution to the War on Terror.” “But there is no example where a president asked for funding for something from Congress, Congress said, ‘No,’ and the president said, ‘I’ll use emergency powers to do it anyway.’”…

Lawmakers could, for example, impose a strict definition of what qualifies as an emergency, taking away presidential flexibility to deal with unforeseen circumstances. Congress could also attach “sunset” clauses to emergency statutes, so that the president’s special powers would automatically deactivate after a month or two without new action by lawmakers to extend them.