Contrary to continuing Left Myth here and elsewhere, the POTUS ABSOLUTELY CAN declare a national emergency at the border, declare a WALL AS THE SOLUTION, and divert funding from other Executive Branch areas to pay for it:
Yes, The President Can Declare A 'National Emergency' To Build A Wall
The president is correct
Some scholars of presidential emergency powers say there is next to nothing, at least procedurally, that Capitol Hill could do to stop Trump from exercising what lawmakers of all stripes agree is his right to declare a national emergency.
"Congress chose not to put any substantial — or really any — barriers on the president's ability to declare a national emergency," says Elizabeth Goitein, co-director of the Brennan Center for Justice's Liberty and National Security Program.
"So if he can really just sign his name to a piece of paper, whether it is a real emergency or not," she adds, "that creates a state of emergency that gives him access to these special powers that are contained in more than 100 different provisions of law that Congress has passed over the years."
Trump has already invoked national emergency powers on three occasions, adding to the 28 earlier national emergency measures that remain in effect.
https://www.npr.org/2019/01/09/683501440/congress-aims-to-control-presidents-emergency-powers
~From the National Emergencies Act of 1976 (amended in 1985):
The Act authorized the President to activate emergency provisions of law via an emergency declaration on the conditions that the President specifies the provisions so activated and notifies Congress. An activation would expire if the President expressly terminated the emergency, or did not renew the emergency annually, or if each house of Congress passed a resolution terminating the emergency. After presidents objected to this "Congressional termination" provision on separation of powers grounds, it was replaced in 1985 with termination by an enacted joint resolution. This means that for Congress to rescind a declared emergency, not only must they pass the joint resolution, but the President must sign the legislation. The Act also requires the President and executive agencies to maintain records of all orders and regulations that proceed from use of emergency authority, and to regularly report the cost incurred to Congress. ~
https://en.wikipedia.org/wiki/National_Emergencies_Act