I'll just comment that it's pretty sad when anybody who doesn't want taxpayer support of religion is dismissed as a "leftist".
I didn't intend the term "the left" as a dismissal, and I don't understand why you think I did. I agree that "left" and "right" are very broad-brush concepts, but I think they can be useful nonetheless. In any case I lean left on many issues myself; this happens to be one where I disagree with the mainstream left, and I think it's an interesting one.
First, a lot of vouchers would end up subsidizing religious education. Previous posts have covered this issue.
I addressed it too, you may notice. I'm not denying there will be some of that. What I question is people's priorities when they say it's more important to prevent that than to have successful, effective schools. I would happily accept a few more parents sending their children to religious schools (as some already do) in order to have the Jaime Escalantes of the country teaching calculus and other hard subjects to many thousands of kids who currently never have a chance to learn them. Doesn't that sound like a good trade to you too??
Please read TFA, if you haven't already, or read it again. Look at the incredible damage that's being done by the current system.
I agree that charter schools are a step in the right direction, but I don't think they're enough. The power of the teachers' unions has to be broken.