To be clear upfront, I am going to state the obvious: I think Trump's proclamation of a "National Emergency" (sic) is galling, pathetic, stomach-churning,racist, divisive, autocratic, and I assume wildly unconstitutional and illegal, and fill in the rest of the like-adjectives you can think of.
That out of the way, I am also not overly-concerned by the impact of it. (I don't mean the precedent it would set, but rather the immediate results.) And in some ways is a good thing. Let me explain.
For the sake of argument, let's assume that all challenges to this fail, that it gets through the courts, beyond the Supreme Court, and every effort to stop it in Congress, and he gets his money. The most money that Trump will get is $8 billion. Now, of course, that's a lot of money that would be take from areas that need it, including bizarrely, national defense, and which will cost the taxpaying public. But it's a paltry amount as part of the national budget, and it's nothing he can really do much of anything with. Keep in mind that Trump was once offered $25 billion for his wall which he turned down because it was not nearly enough. Some studies say that $50 billion would be needed to build what Trump is talking about. Perhaps even more. So, $8 billion will not come even close to building his "wall" or accomplish anything of substance. All it will do, the only thing, is placate his base. A base which already adored him and believes he could do no wrong. If there was no wall built, that base would still adore him, blaming its absence on Democrats, a weak Republican Congress, the courts and whatever other targets they could think of. Anyone, anything but Trump. So, while the only thing this $8 billion would do (again, assuming he actually gets it) is placate a base that didn't need placating.
On the other hand, it is going to cause massive damage to Trump himself in other ways, and to the Republican Party.
Even if Republicans in Congress vote for it, they will hate that they feel they have to. We know this because, when the GOP controlled both houses of Congress and the White House they never passed a bill for a wall. So voting for it now only means they feel obligated to do something they're against. All because Trump put them in that hated position. They know it's idiotic, they know there's no "emergency," they know this is not needed. (After all, we know that, again, because Trump just told us all in the Rose Garden.) So, this will cause a major rift between Trump and Republicans in Congress.
Further, I feel certain that the public will upset by this and blame Trump -- and the GOP for enabling him. We know this because polls show that the public is strongly against a wall. And also we know that the public voted for Democrats to take control of the House of Representatives despite Trump not only campaigning about the threat of marauding Hondurans coming to take over the U.S., but also sending actual troops to the border. They knew there was no "emergency." So, to see Trump circumvent the Congress by calling for a "National Emergency" (sic) and taking money from national defense can likely only outrage the public and lose Trump -- and the GOP which enabled him -- support.
And this further will push a wedge between Trump and the Republicans in Congress, who see the looming 2020 elections when all the House is up for reelection and Republicans in the Senate this time around are the ones most at risk. Not only will that make it more difficult to get any legislation through, but when the Senate is the body that votes on impeachment, I'm not really sure if you want to risk pissing off your only means of support. And they will all know that this set a horrific precedent for allowing the president to abuse authority around the responsibilities of Congress.. (Though in fairness, that being the case, enabling Republicans in Congress would have no one to blame for that but themselves.) But precedent is a separate matter from what paltry detritus Trump has to gain from his action, and how much he has to lose.
And all this is based on the assumption that it all goes through and Trump gets his money for his wall. There's almost nothing to gain from that, and a huge amount to lose.
Now imagine if he doesn't get it. That he's stopped by the courts or Congress. That may even be worse. Yes, Trump will have lots of people to blame, and his base will love it. But they'd love him regardless. And everyone else will be infuriated that they had to go through this autocratic madness. And it will be a humiliating loss.
To repeat what I said at the beginning: I think Trump's proclamation of a "National Emergency" (sic) is galling, pathetic, stomach-churning, autocratic, and I assume wildly unconstitutional and illegal, and fill in the rest of the like-adjectives you can think of. And I hope it gets stopped cold at every possible corner where it can be, in the most blunt, devastating way.
But it's a no-win situation for him whatever happens.
Now, if you'll excuse me, I'm going to take my cue from Trump and go on vacation to play golf...