His whole appeal is the fact he's going do what he said. He ain't no politician who lies to get into power then drop what he/she said. That's Trump in his eyes. By doing this he's already guaranteed support for 4 years time. He'll keep pushing and pushing. Whether that is childish or not I suppose it could be but the more Mexican president resists the more it plays into Trumps hand as will only motivate his core to back him more. The next 4 years will be a vicious cycle.