Re(12): On Trump's Declaring a National Emergency over the Border wall Posted on 2/2/2019 at 18:27:32 by Bill Crawford, (14)
The most likely basis for a court challenge to Trump's Declaring the border wall a national emergency appears to be the likelihood of his re-directing military construction funds for the construction of the wall.
The below-linked article discusses this issue in a pretty thorough manner.
As I read it, it looks like Trump's re-direction of military construction funds would be upheld in any supreme court decision.