Presidential Emergency Power: Can Trump Build a Wall?

The more President Trump changes his mind about declaring a national emergency to build his wall the less likely it is that he has the authority to do it.  But even if he had declared a national emergency immediately, it is unlikely the Constitution or congressional statute allows him to do it.  That is perhaps why Trump has not invoke emergency powers to build the wall–basic principles of American law suggest he lacks the authority to do it.
The US Constitution is a power conferring document.  There are no extra-constitutional powers.  Before the president or any branch of the national government does something it needs to trace authority back to the explicit text of the Constitution or the power must be necessarily implied.  In the case of the president, his authority comes from Article II of the Constitution, or he may be delegated some additional authority to him from Congress via Article I.
Under Article II section 1, executive power is vested in the president.  Under Article II, section 3, the president shall take care that the laws are faithfully executed.  Through either of these clauses the president can issue an executive order and declare a national emergency, but what does that really mean?  Does it mean presidents can ignore existing law or do whatever they want for whatever reason?  Doubtful.  President’s cannot manufacture an emergency and then invoke undefined  powers to ignore the law or the Constitution; this idea violates the very idea of rule of law and the concept of American constitutionalism.
Presidents do not get the right to decide what is an emergency and do whatever they want.  The Supreme Court said this arguably one of the most important cases of all time, Youngstown v Sawyer, 343 U.S. 579 (1952).  Here President Truman declared an emergency and nationalized the steel mills during the Korean War to prevent a labor strike that would have affected production.  If ever there was a crisis, war was it, and the Court ruled against Truman.  In Youngstown many legal scholars think Justice Jackson’s concurrence is best statement ever on presidential power.  Acting with congressional assent presidents have  enhanced power, with congressional silence they only have the powers given to them by Article II, and acting against congressional wishes, presidents have diminished authority.  Truman was acting on his own and lacked the authority to act.  However this third scenario is where Trump is right now when it comes to the wall and the emergency here is no greater than what Truman faced.  There are at least two laws that limit Trump’s discretion to act.
The first law is the 1974 National Emergencies Act.  Responding to abuse by previous presidents, this Act sought to constrain unregulated use of presidential declarations of national emergencies.  Unfortunately, the Act does not define what a national emergency is, leaving it up to the president’s discretion, subject to congressional veto or check on the declaration.  However, there are three problems with this Act. 
First, the original concept of this legislative veto is arguably unconstitutional  as a result of other Supreme Court decisions such as INS v. Chadha, 462 U.S. 919 (1983).  Second, it is not clear that Congress can give the president unbridled or unrestricted discretion to act, especially to disregard  other congressional statutes or provisions, unless it specifically says so.  While the Court is reluctant to invoke the delegation doctrine to limit presidential authority and has not done so since the New Deal, the wide-open language of the National Emergencies Act is a perfect place for the Roberts Court to do this.
But third, there is another law that points to why the president cannot invoke the National Emergencies Act to build the wall.  It is the 1974 Budget Act.  Responding to abuses by Richard Nixon who impounded or refused to spend money allocated by Congress, the Act placed limits on the ability of the president to do this.   In Train v. City of New York, 420 U.S. 35 (1975), the Supreme Court upheld the Budget Act saying that once Congress has allocated money for something the president is not free to disregard the law and spend or not as he wishes. 
The question is, does the authority grant to the president under the National Emergencies Act supersede the Budget Act?  There is no indication that the latter was meant to free the president to disregard spending priorities mandated by Congress.  If there were, Congress would have said so in the Act.  But even if such an intent or purpose was implied, it is doubtful given the purposes of the Budget and National Emergencies Acts that these statutes could be interpreted to be expanding as opposed to contracting presidential authority.  But, as noted above, if Congress did intend this,  it is unconstitutional.  Finally, supposedly Trump is looking at raiding money from hurricane  relief funds.  There is no indication that in allocating this money Congress intended it to be diverted for other purposes.  Whatever general intent the Emergencies Act has, it cannot override specific intent or a mandate to spend as decided by a particular statute. 
There are two addition  problems with Trump invoking the National Emergencies Act.  First, the Act is not for politically manufactured emergencies, especially ones created by the president himself.    Trump today could end the emergency by agreeing to sign a spending bill or continuing resolution.  Giving presidents the ability to invoke a national emergency during negotiations with Congress so that they can get their way is like letting someone take their bat and ball and going home if they are losing.  The National Emergencies Act is not a tool to bypass the Constitution.   Under Chevron v. NRDC, 468 U.S. 1227 (1984), courts are supposed to give deference to reasonable executive agency determinations of ambiguous statutes.  Conservatives, including Justice  Gorsuch, have increasingly questioned Chevron deference and there many be a majority either unwilling to give Trump the benefit of doubt regarding what a national emergency is, finding his use  or interpretation is unreasonable.
Second,  the longer and more Trump delays or vacillates in declaring an emergency the more it shows that the border problem is not an emergency in any ordinary dictionary meaning of the term.
Perhaps then why the President is not invoking the National Emergency Act simply is that  he has been told he will not win this legally, on top of the fact that he is not winning this fight politically.