Through some combination of confusing lies, Democrats have made this point obscure and non-obvious.
The reality, of course, is that poor people tend not to show up to vote, and when they do, they reliably vote for the people promising free stuff. This makes them reliable Democrats.
And teacher’s unions, knowing that some percentage of their membership does not belong in front of children because they are lazy, incompetent, abusive or otherwise unfit, want to make sure they are never exposed to competition.
Here is the thing: good teachers need not fear competition. The goal is not to run out good teachers, but bad ones. The goal is to REWARD good teachers and fire bad ones. I personally would support much higher salaries and much better benefit packages for teachers, if I could be assured there was some structural incentive, like competition, for them to reliably produce good work over the long haul.
In Sweden–where they seemingly performed this logical operation, reached the correct solution, then made the morally defensible decision–they found that charter schools not only worked to improve educational outcomes among the poor, but that they reliably improved the quality of all schools around them. Competition will do that.
[And I will note that competition assumes that only self interest is, in the long run, a reliable motivator. Morality and character are fine, but they decay over time, and no means has yet been found to ensure they are reliably produced–other than, perhaps, a top quality education, which is the topic being discussed.]
Logically, then, the only rational goal being defended by the unions is their own existence. Union bosses don’t want to be fired either, so their JOB, by and large, is to protect the incompetent and unfit. That is literally the only solid reason they have for existing.
The union bosses, in turn, see Democrats as their own protectors, and so contribute heavily to them, like all public sector unions, which also exist mainly to protect the incompetent and unfit.
Thus, all Democrats who oppose vouchers and charter school are literally protecting incompetent and unfit teachers simply because it nets them campaign cash, and they are literally standing in the way of poor kids getting access to the best possible education available to them. They are helping ensure, in other words, another generation of ignorant and thus poor kids; and thus another generation of people who want the public largesse Democrats have positioned themselves to dole out, which is never enough to live a good life, but enough to support bad decisions and chronic underwork.