There are a bunch of problems with Trump’s threat, starting with its incoherence: If he can use emergency powers, why is the government shut down?
Beyond that, there are constitutional limits to what Trump can do. Certainly, there are dozens of laws that give the president power to do certain things (e.g., take over media outlets, deploy troops within the United States), which in the Trump presidency — and for future presidencies that might be as unfit and authoritarian-minded as this one — Congress should review and modify or repeal. (Of course, even if individual statutes authorized the president to take certain actions, it’s not clear Congress even has the power to delegate core functions (e.g., spending billions of dollars) to the executive. Regardless of whether they say such actions are not subject to judicial review, they are.)
Statute or no statute, a president trying to exercise such sweeping power after failing for two weeks to get Congress to authorize funds (a really bad fact against Trump if he were to claim he inherently had such power) would quickly run up against clear precedent.
The courts, as happened in the Youngstown Sheet & Tube Co. v. Sawyer case, in which President Harry S. Truman was prevented from seizing steel mills during wartime, are very likely to look askance at power grabs, especially when there is no war and no actual emergency. In Youngstown, the Supreme Court held:
The order cannot properly be sustained as an exercise of the President’s military power as Commander in Chief of the Armed Forces. The Government attempts to do so by citing a number of cases upholding broad powers in military commanders engaged in day-to-day fighting in a theater of war. Such cases need not concern us here. Even though “theater of war” be an expanding concept, we cannot with faithfulness to our constitutional system hold that the Commander in Chief of the Armed Forces has the ultimate power as such to take possession of private property in order to keep labor disputes from stopping production. This is a job for the Nation’s lawmakers, not for its military authorities. …The Founders of this Nation entrusted the lawmaking power to the Congress alone in both good and bad times. It would do no good to recall the historical events, the fears of power, and the hopes for freedom that lay behind their choice. Such a review would but confirm our holding that this seizure order cannot stand.
In the case of the wall, Trump would have to seize acres upon acres of private property (particularly in Texas), so litigants (many Republicans) would abound.
In sum, there is no emergency and no constitutional authority for Trump to do this on his own. Nevertheless, perhaps the House should pass a resolution declaring that any such action would be unconstitutional, a gross violation of the president’s oath of office and grounds to commence impeachment. The House should also put Homeland Security Secretary Kirstjen Nielsen under oath to testify about the actual number of border crossings, the lack of evidence any terrorist has crossed the southern border, the origins of the inhumane family-separation policy and the conditions under which two migrant children have died. If there is an emergency, it is one caused entirely by Trump’s ignorance, hysteria and vanity.
That Trump would even suggest such an unconstitutional move tells us a lot about his B-team administration (more like D-team) and the state of the Republican Party. It wasn’t too long ago that Republicans such as Sen. Ted Cruz (Tex.) and anti-immigrant activists denounced President Barack Obama for the executive order launching Deferred Action for Childhood Arrivals. (“President Obama has stretched his office’s executive powers well past any traditional or historical limits when the country is not in the midst of a severe crisis,” wrote Stanley Renshon of the Center for Immigration Studies.)
Fast-forward to 2019, when Republicans now defend Trump or mutely stand by as he makes grand pronouncements of unlimited power. Republicans' passivity underscores their constitutional illiteracy and lack of will to carry out their own oaths of office. Senate Majority Leader Mitch McConnell (R-Ky.) better get back to claim his job before the president erases Article I from the Constitution.