Incidentally, Megan's post about the gross ignorance or dishonesty of her ideological comrades in arms ends up in one of her signature bizarre overstatements of the influence of teacher's unions on progressive politics in America: "The Laffer Curve and the supply siders pushing it seem to be the teacher's unions of the right."
Obviously, teacher's unions are an influential group. That said, the No Child Left Behind education reform that AFT and the NEA very much don't like was primarily written by Ted Kennedy and George Miller, who aren't just Democrats, but actually chair the education committees of the House and Senate. The American Prospect tends, in my view, to bend over backwards to be friendly to the teacher's unions, but never spiked any writing I did that went against the union line. The liberal Washington Monthly ran a 2004 article lauding John Kerry (at the time, an influential Democrat) for bucking the unions on key reforms:
Many liberals had hoped that Kerry would attack the testing requirement set forth in Bush's No Child Left Behind Act (NCLB), which has become increasingly unpopular, especially among teachers' unions. But Kerry, who had voted for NCLB, instead challenged two longstanding, and fiercely defended, union prerogatives: seniority-based pay increases and rules virtually guaranteeing veteran teachers tenure. The candidate proposed a "new bargain"--a $30 billion, 10-year plan of federal grants which would allow districts to raise the pay of teachers whose students consistently test above average, while at the same time making it easier for schools to fire bad teachers. "Greater achievement ought to be a goal," Kerry said, "and it should be able to command greater pay, just the way it does in every other sector of professional employment."
That article didn't get spiked and Kerry is still a Democrat in good standing. Robert Gordon, known to pen such things as "That Republicans are fond of making these points--and unions and school officials are not fond of hearing them--does not make them less true" is a senior fellow at the Center for American Progress and also served as domestic policy director for the Kerry-Edwards campaign.
Basically, the unions, while influential, also see their influence checked by counterbalancing interest groups within the Democratic Party, including the major civil rights organizations, several major charitable foundations, and the proclivity of various wealthy progressives to fund operations like Democrats for Education Reform. There's absolutely no comparison between this and the ways in which supply-side orthodoxy dominates the right. You see prominent Democratic politicians, progressive media outlets, and think tanks all deviating from the union line while you almost never see the right's institutions break publicly with the Lafferites.