Trump will be resoundingly defeated in court if he declares a national emergency (because there is none).

Check out what one federal judge has already said about this:

[quote]
In a ruling denying the government’s bid for more time, U.S. District Judge William G. Young said lapses in federal appropriations, like the current one triggered by President Donald Trump’s demand for funding for a border wall with Mexico, aren’t a government “policy” that could theoretically justify staying such a lawsuit.

“Let us talk plain -- they are simply an abdication by the president and the Congress (which could override a presidential veto) of the duty to govern responsibly to the end that all the laws may be faithfully executed,” Young said in the Jan. 2 ruling in San Juan. “Nor does such a lapse in any way excuse this court from exercising its own constitutional functions.“

Young, who is based in the Boston federal court but sitting in San Juan as a visiting judge, sarcastically compared the situation to a major corporation that “for whatever reason” decided not to pay its attorneys involved in pending litigation and instructed them not to interact with the court.

“Then the corporation says to the court, ‘We greatly regret any disruptions caused to the court and to other litigants, but please stay all proceedings until we get our act together.’ This does not constitute ‘good cause’ for any stay,” Young wrote. “In fact, it is laughable.“[/quote]

https://biglawbusiness.com/judge-says-us-laughable-for-using-shutdown-to-delay-suit-1-1

There is also legal precedent. President Truman tried to seize control of steel mills that were on strike as a national emergency. The Supreme Court denied his actions. That was much more a crisis than Trump's manufactured crisis, and will almost certainly be struck down if he attempts it.

https://thehill.com/opinion/judiciary/424314-yes-trump-has-authority-to-declare-national-emergency-for-border-wall