Why Do I Hope President Trump Declares a National Emergency to Build a Wall?

Because he will likely lose before the U.S. Supreme Court—and along the way, he might be embarrassed.

Why will he probably lose? 
The lead case on executive orders is Youngstown Sheet & Tube Co. v. Sawyer (1953). In the midst of a “hot war” (real fighting involving U.S. troops in Korea), President Truman ordered a seizure of steel plants in order keep munitions flowing to the military. Unions were ramping up a crippling strike in the steel industry when Truman sought to keep the plants open. Believe it or not, seizures were very common in WW II—and no doubt, Truman was hoping the Supreme Court would equate the Korean War with WW II and uphold his authority. The Court said no to Truman— you have exceeded your constitutional limits. Only if the current Supreme Court equates the situation on the U.S.-Mexico border to WWII, there is no chance Trump will win.

But there is more.
Watch for President Trump to put on his Commander in Chief hat in making the order-- then read what the Supreme Court said in the Truman case:

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.

Who would sue? Two likely candidates are the U.S. House of Representatives, and a citizen taxpayer. Each would have different claims. The House would argue that the President has violated the doctrine of separation of powers and undermined the will of the people as reflected in the Continuing Resolution passed two days ago in that chamber. Unless a majority of justices suspends our constitutional system, it’s hard to see how Trump wins that one.

Then there is Fill-in-the-Blank citizen. She will make the argument that tax dollars are being diverted for an unconstitutional purpose via an unconstitutional process. Again, I rate the odds of a successful challenge as close to 100%.

Along the way, President Trump would encounter heavy head winds.

First, in a lawsuit a plaintiff would demand discovery of documents, emails, and such. The lawsuit would open the White House to intrusive information demands. Relatedly, the President would be deposed. This happened to President Clinton when Paula Jones sued him a civil matter, alleging sexual misconduct. And the President would be under oath. Given his propensity to lie, this would be a perjury trap entirely of his own making.

The executive branch has accumulated vast powers since 9/11. That was a true and extraordinary national emergency. But within a few years, that authority was used for a wide range of other purposes—intervening in Iraq (Bush II), and massively regulating banks (Obama), to name two contrasting examples. It is time for our presidency to be a coordinate actor in government. After all, we revolted against Britain because King George III abused his powers. Bring it on.
Caveat: Yes, I could be wrong. If that happens, our constitutional system of checks and balances will be seriously eroded. In addition, our Supreme Court will risk losing its legitimacy as a neutral body of government (though people on the left and right believe with some justification that this has already happened).