I sort of share the concern (though I doubt I agree about how to deal with the problem), but how many people know that the groups who donate the most money in America are teachers unions? I doubt many do. And I doubt that those who do know are especially concerned about this fact. Here's the heroic Will Wilkinson
If you believe, as I do, that the returns to further government spending on education, given its present structure, is zero or negative, and that the best hope for increasing the quality of education for the least well-off, and for increasing economic and social mobility generally, is to legalize competitive markets in education, then you will tend to believe, as I do, that this attempt to destroy voucher programs before than can show themselves effective is nothing less than a powerful political interest group screwing over poor people by bending the democratic process to their advantage. The sad thing, from my perspective, is that strong Democratic partisans (and especially members of the teachers’ unions) are likely to violently reject any such argument out of hand on the basis of their deep conviction that the Democratic Party cares about the poor, and so would certainly not allow itself to become captured by groups with interests diametrically opposed to interests of the poor.