With Democrats in Congress refusing to appropriate $5.7 billion for a border wall, President Trump now says he may declare a national emergency. He’s probably right that he has the legal authority, but it would set a bad precedent that conservatives who believe in the separation of powers could live to regret.

Mr. Trump may declare an emergency that would let him reallocate funds that Congress has appropriated for military construction to build his wall. Before visiting the border on Thursday, he warned that if negotiations with Democrats don’t “work out, probably I will do it. I would almost say definitely.”

An emergency declaration could let Mr. Trump end the shutdown without conceding an inch to Speaker Nancy Pelosi, but it would strain the limits of his executive authority.

Presidents over the decades have invoked emergency powers, sometimes grounded in law though often not. While a President may sometimes need to act with dispatch, abuses of executive power prompted Congress to pass the National Emergencies Act of 1976. The law requires the President to activate his powers under one of 130 or so statutes that authorize executive emergency actions.

Members of Congress with a two-thirds vote may revoke an emergency declaration, but the President otherwise enjoys wide latitude within his statutorily delegated powers. Mr. Trump’s advisers have cited a statute that allows the Defense Secretary during a national emergency “to undertake military construction projects, not otherwise authorized by law that are necessary to support such use of the armed forces.”