He needs to add that the deportation may not be to their country of origin.
Honestly an effort of that large scale can't possibly sort everyone out, 5 million from mexico, 5 million back,1 million from Gaza, 1 million back. . . . Since they are undocumented, we have no way of knowing where they came from. Gaza to Mexico, Ukraine to Gaza, Mexico to Ukraine, it really doesn't matter to us who goes where. Those that are here to vote illegally might think twice if they come to the realization that the worst case is not that they will return to where they come from.
Send them back over the border in the area they crossed in the first place and tell them to get the fuck home on their own from there, then close the wall.
Problem solved. Mexico helped many of them get here so they can help them get back as well. Or whatever.
That train they were all sitting on top of goes both ways!