DENVER – The Obama administration’s strained relationship with teacher union activists reached a new low late last week.

On Independence Day, rank-and-file members at the National Education Association’s annual convention passed a “new business item” – a kind of resolution – calling for Arne Duncan to resign his post as U.S. education secretary, Education Week reports.

NEA members cited Duncan’s support of expanding charter schools, increasing standardized testing, and tying teacher evaluations to students’ test scores are reasons why he should step down.

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NEA members had considered – and rejected – similar resolutions against Duncan during their four previous conventions. What tipped the scales against Duncan this time was his mealy-mouthed support of Vergara v. California, a recent judicial ruling in which teacher tenure and other job protections were found to violate students’ constitutional right to a quality education.

“Vergara was the straw that broke the camel’s back,” California Teachers Association President Dean Vogel told Education Week. “The Secretary’s response to the Vergara verdict—it was just shameful. And it underscored his lack of understanding.”

NEA President Dennis Van Roekel seemed to downplay the significance of the vote by offering some banalities about union members’ frustrations and pledging to “push” the Obama administration to pursue “student-centered policy changes.”

Clearly, Van Roekel understands that an open civil war between NEA members and President Obama’s hand-chosen education secretary isn’t going benefit the Democratic Party – the union’s party – during an election year. Things are already looking bleak for the Democrats; November could be really ugly for the party if NEA members decide to sit on their hands and wallets instead of helping with the various Senate and gubernatorial campaigns.

It will be interesting to see how the White House responds to the union’s growing dissatisfaction. Should Duncan soon decide he wants to spend more time with his family, that’ll be an indication that the Obama administration is genuinely concerned that it has lost the fervent support of unionized teachers, one of the president’s most important backers.