He basically did that, last October, with the active duty mobilization to the border. It fell flat, along with his phony promise of a 10% middle class tax cut.
He mobilized the military to the border in order to 'assist' border patrol with the incoming caravan(s). He also agreed to allow the U.N. to decide who was granted asylum and what country (including the U.S.) they were going to be placed. Our military was in part there to assist in processing paperwork.
That wasn't the first time a caravan mob had reached our borders only in that case and those following there were thousands rather than hundreds.
It would seem to me that would have been a very opportune time to declare a national emergency if he was going to declare one.
Again, his push for a border wall, should have been long ago.