Let's play Dr. Seuss for a moment. Imagine that it's a dry, hot, hot, hot day in the land of Amercia, and you have a really big fish tank outside in the sun, full to the brim with sad fish jammed together in this big tank with barely enough oxygen to breathe and no room to move. Along comes the fix-it-up chappie with a big cup to scoop out any fish that want to get out of the tank and put them in a better tank with more room, better fish-teachers, better fish food, and in the shade. The only problem is that it takes a long time to get to that other tank and fish don't own cars. Even worse, when and if they finally arrive, the fish in that other tank will not happy to have new fish crowding them.
So, even with this understanding, many fish volunteer to get scooped out and transferred to the new tank. The chappie scoops fish after fish into a large paper cups and sets them on the ground outside the tank. Magical friendly pelicans then scoop them up and fly them to the new tank. Sure some of the water and a few of the fish are lost along the way, but who cares, these fish are going to a better tank. When they arrive at this tank, The fix-it-up chappie dumps all the fish and their water into the tank until it overflows with water and is almost as crowded as the original tank. Now all the fish in this tank are unhappy and violence breaks out in the tank!
While the fish are killing each other in the new tank, let's go back to the old tank. The fish who decided to stay now only have half the water that was originally in the tank, and the hot, hot, hot sun is evaporating it at an alarming pace. There's more room in the tank, but now there isn't enough water to support the teachers and students. None of the fish in this tank are happy either, and now there is no place for them to "choose" to go, and not enough water for them to survive. Eventually, all the fish in both tanks are dead. Thanks a lot!
While this scenario may be a little extreme, it is, nonetheless based in the reality of the limitations and dangers of attempting to implement a voucher system in education. In essence, a plan like this does nothing except damage education, further weaken already struggling schools, and tax "better" schools to the point where they will cease to be good schools. Ultimately, this will cripple and destroy our education system, making it the "3rd World" institution that fix-it-up Mitt was criticizing.