President's Oath of Office:
US Constitution, Article II, Section 1
Before he enter on the execution of his office, he shall take the following oath or affirmation: "I do solemnly swear (or affirm) that I will faithfully execute the office of President of the United States, and will to the best of my ability, preserve, protect, and defend the Constitution of the United States." |
The President's first and primary duty is to protect and defend the Constitution. All his other duties derive from said Constitution and thus none of them can exceed this document in priority or legality.
In the federal government of the United States, the power of the purse is vested in the Congress as laid down in the Constitution of the United States, Article I, Section 9, Clause 7 (the Appropriations Clause) and Article I, Section 8, Clause 1 (the Taxing and Spending Clause).
The power of the purse plays a critical role in the relationship of the United States Congress and the President of the United States, and has been the main historic tool by which Congress has limited executive power. |
https://en.wikipedia.org/wiki/Power_of_the_purse
On 15 Feb, Congress passed a bill setting the specific amount of funds that may be spent on a border wall and sent it to President Trump who signed it. Congress acted according to its Constitutional authorities.
On 15 Feb, President Trump declared a state of emergency with the specific intent of defeating the above limits, thus violating the checks and balances built into the Constitution by the power of the purse.
That the Emergency Powers Act may have been poorly written and technically construed to say that Trump has this authority is irrelevant. The authority of all federal law derives from the Constitution and no federal law or bill less than an amendment can override the Constitution.
This action puts President Trump in violation of his oath of office. And any Republican Congressman who supports his action when the Congress acts to correct the situation will be in violation of their oaths as well.