THE NATURE OF SCHOOL CHOICE, as explained at Atlantic Blog.

Vouchers are too big a danger not only to the teachers unions (of which the Democratic Party is a wholly owned subsidiary), but to a bipartisan coalition of well off people. Whatever would happen to their property values if their schools ceased to be the exclusive property of their wealthy suburbs and became open to just anyone?

A Scitovsky compensation in which residents of such districts receive compensation from poorer people allowed access to their schools by the voucher is a tough sell. Mortgage qualification standards thus become the method by which people act on their school choices.