It seems Trump’s side could argue that congress has already approved of the Wall being built as Congress just funded $1.3 billion for building it.
I predict that a liberal judge on some District Court will declare the Emergency Unconstitutional, even if the judge has no grounds to do so. You know why? Because there is no downside to the judge and only upside. The upside is the judgement will immediately make the news and drive the news cycle and provide wall to wall talking points for liberals. Even if it is eventually overturned, libs will keep yelling about unconstitutional actions by Trump.