Here is a WAPo article about the constitutionality of Trump's declaration and the Wall, written by a Con Law Prof at NYU Law named Richard H. Pildes
https://www.washingtonpost.com/opin...rations/?noredirect=on&utm_term=.ef5d5d8c1941
Pildes notes the National Emergencies Act (NEA), enacted l976, has been used by presidents 58 times. In each case, the president (both Ds and Rs) spent funds not appropriated by Congress and the Supreme Court did not overturn the action.
“Courts are uncomfortable when asked to second-guess presidential judgments in areas such as national security, foreign affairs and emergencies.”
Pildes cites Immigration and Naturalization Service v. Chadha as a key case for the idea that -- “legislative vetoes are unconstitutional” -- including vetoes of actions under the National Emergencies Act.
“Congress cannot act through a legislative veto but can act only by passing a new law.”
INS v. Chadha, 462 U.S. 919 (1983)
Chadha gives
"Trump the chance to veto Congress' disapproval ... ”
of his actions. A Trump veto means Congress would need a “two-thirds majority in each chamber to override.” Such a 2/3 supermajority would be 67 votes in Senate and 290 votes in House. This is not going to happen in either chamber.
Lastly, the final advantage for Trump is that the law in issue does
“does not define what constitutes an emergency.”
So while some in Congress may be unhappy, it is their own fault, something they usually conveniently overlook.
Thus President Trump neither violated the Constitution nor violated separation of powers. His unilateral action was a constitutional power ceded to him by an Act of Congress (the NEA in l976) and used by presidents on both sides of the political aisle 58 times. Obama, for example, used the act to transfer funds without congressional authority to his health care act.
So if this does end up before the SCOTUS, which seems likely, the vote, following their own precedent in Chadha, should be 9-0. I doubt this will happen though since liberals on the Court sadly stick together as a block for purely political reasons regardless of the law (like Pelosi's House, its just what they do). So it smells like another 5 to 4 vote. But the outcome is inevitable.