This is really a slam-dunk for the GOP if they push it in the fall. Who in their right mind is against enforcing existing laws?
Democrats. Along with Bush, McCain, Graham...
Two points you overlook. The GOP does not want to control the border. You only need look at the history of the last 30 years to see this.
Second, even now with overwhelming public support for closing the border, it has been only lip service and panders to voters by the GOP.