The current situation in Washington is a slow-motion Reichstag fire.  We need to stop it, immediately.  The original 1933 Reichstag fire is infamous, because it was used as a pretext for passing laws that allowed the Führer to rule by fiat.

The lesson for us is clear: nobody should be allowed to rule by decree.  It is important not to have any laws on the books -- old laws or new -- that allow somebody to use a trumped-up emergency as a pretext to do whatever he wants.  Existing law(*) needs to be fixed, because it grants the executive wide latitude for dealing with emergencies.  It assumes the president will be honest and honorable, which is no longer a valid assumption.  The best way to proceed is to attach an "anti-emergency" rider to the pending budget legislation.

To put things in perspective, note that there are three issues on the table:
  1) In general, "emergencies" must never be used as a pretext for rule by decree.  This is the core issue.  This is essential to the survival of democracy.
  2) As a matter of policy, there should be no wall at all.
  3) As a corollary of (1) and/or (2), there should be no "emergency" wall.

Although the wall issue is important, the core issue is even more important.

If things continue on their present course, it is very likely that you-know-who will declare an emergency and start building the "wall", whereupon a great many struggles and sacrifices -- including the five-week shutdown -- will have been for naught.  As bad as that would be in the short run, the long-term damage would be even worse.  It would embolden the bad guys.  It would give them the idea they can rule by decree.

This is an eminently fixable problem.  Congress's power of the purse is essentially absolute: «No Money shall be drawn from the Treasury, but in Consequence of Appropriations made by Law».(**) The existing emergency powers law was passed by Congress and can be repealed or amended.

Congress is "supposed" to pass a budget before the 15th.  All the action is in the bicameral conference committee, which is happening now.  The rules make it exceedingly hard to amend a conference report (understandably enough).  There is essentially zero chance of passing the anti-emergency provision as a stand-alone bill, so a budget rider is needed.  Delaying action on this issue is tantamount to surrender.

Including such a rider might make the budget harder to pass, because it means the bad guys will lose face.  AFAICT the current plan allows everybody to save face: (a) Congress passes a budget, pretending they are not funding the wall; then (b) an emergency is declared, and (c) Dems get to make a big stink about it.  IMHO that's a terrible idea.  Basic notions of good governance tell us that Congress should make an unambiguous decision on the issue: either approve the wall or forbid it.  Don't pretend.

In particular, pretending is terrible in terms of policy /and/ in terms of politics.  It makes Congress look both weak and foolish.  It makes the #fakepresident look strong, hands him an issue that he thinks is a winner for him, and allows him to drag the issue out for years.

If the budget bill can't pass with the anti-emergency provision, it shouldn't pass at all.  This is what the previous shutdown was about.

Similarly, relying on the courts is terrible in terms of policy /and/ in terms of politics.  It's not the the courts' job to decide what's an emergency and what's not.  And it's not at all clear what the final decision would be; usually (albeit not always), courts defer to the executive on claims involving security and emergencies.  Even more importantly, litigation should be seen as the final safety net.  Safety comes from having N redundant layers.  In contrast, if you intentionally throw away N-1 of the layers and rely on the last backup, it's no longer the backup, and you've forfeited almost all of your safety.

It's hard to say exactly what the legislative language should be, but roughly speaking here is a common-sense three-part definition of an emergency:
 a) dire
 b) sudden
 c) urgent

This is consistent with the definitions used elsewhere in government and in the real world.(***)  In more detail, a government emergency is a situation where:
 a) there is good reason to believe government action is likely necessary to prevent irreparable harm, and
 b) the situation has arisen so suddenly, and
 c) requires action so urgently, that there has not been any practical opportunity to bring the matter before Congress for consideration.

The administration is authorized to take action that is reasonably necessary and proportionate to deal with the emergency.

As an obvious corollary, any matter that /has/ been considered by Congress is not an emergency, unless things have changed so dramatically that it is effectively a new situation.  Also, a small emergency must not be used as a pretext for disproportionately large action.

These criteria should apply to all emergencies, not just the supposed "wall" emergency.

In any case, remember: We were promised that Mexico would pay for the "wall".  The promises were explicit and emphatic.  No taxpayer funds should be spent on the "wall".


References:
(*)   https://www.law.cornell.edu/uscode/text/50/chapter-34
(**)  https://constitutionus.com/#a1s9c7
(***) https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/instrument_procedures_handbook/media/FAA-H-8083-16B_Appendix_A.pdf