Just something for you to answer:
How do you propose to locate, round up, and detain 12 million people (as a floor figure, keeping in mind the figure is even higher) until you can have them all booted out? How do you tell if someone's here illegally? Do you ask for papers? Issue national ID cards (assuming they cannot be faked)? Do we even have enough prison space? How much to build more (and make sure that is is secure)?
Oh, you need to do this without breaking the bank, getting the wrong people caught up in your enforcement efforts (can't have false arrests now), or doing anything that will look bad on CNN (or any other TV/cable news shows) - this is called acceptability to the American people. That last item is no small issue. If they don't approve, you get your ass voted out of office (just ask the Republicans in Congress).
You not only have to secure the 1,951 miles of border with Mexico, but also the 5,522 miles of border with Canada as well. Not to mention the coasts (East Coast, West Coast, Gulf of Mexico, and Alaskan).
Start running the numbers, and then tell me with a straight face the laws we currently have are enforceable. Put out what you plan to do, and how much it will cost the taxpayers. That means you're going to have to either cut other programs (which have their own lobbies) or you have to raise taxes (note - the last major tax increase cost the Democrats control of Congress in the 1994 elections).
Oh, and there might be other nasty surprises as well. For instance, China has told us they will not accept illegal immigrants we have in custody until we turn over dissidents who we gave given political asylum (in other words, to enforce the law, the United States must betray people whom it has promised safe haven).
Got any answers to those questions?