His popularity would soar. The crybaby media with the sob stories will have no effect, especially if he offers aid to Mexico to shelter, feed and return the migrants. It would be worth it.
But he can absolutely not allow them into this country. The next wave will start almost immediately.
Come on Trump. Show us the balls you have been famous for. Tell what you are going to do and then do it!
In military and security operations, it is best to remain silent about plans (also called "operational security").