Well, Republicans have been running on cracking down on illegal immigration, for years and years.
So all Trump needs to do is call up Ryan and McConnell, and ask them for legislation, to accomplish the withholding.
Easy, right? Then when it doesn't go so well, just blame Trump, right?
Wait, did I miss the part of Republicans following through? That is right, they are too busy, what with recesses, vacations, etc.