The teacher unions are an incredibly important source of money and volunteers for the Democratic Party -- about one in 10 delegates at recent Democratic national conventions have been teacher union members or their spouse. When they snap their fingers, the Democrats jump. Vouchers threaten to dry up dues money, and that is that.
I try to avoid sounding like a partisan, but the fact that the Democratic Party is so tied to groups that harm children (indirectly or directly) is a reason why I will never vote for one of their candidates until the policies they advocate change. On the critical issue of education, the Democratic policy is to keep pouring money into failed schools and failed programs and maybe someday results will improve. It is time for new ideas, so that the next generation after mine (I'm afraid the damage has already been done there) will be prepared to compete in the global market.