Teachers unions have long provided considerable voting and financial support to the Democratic party. But over the last two weeks the top two teacher labor organizations in the United States have aimed bitter criticism not at a Republican public official, but at Democratic U.S. Secretary of Education Arne Duncan.

And in light of those unions' protracted, stubborn resistance to overdue educational reforms, Mr. Duncan deserves an "A" for daring to take on such a major player in the Democratic base. So does President Barack Obama for supporting the secretary's efforts to advance meaningful changes in America's public school system.

Anybody who doubts Mr. Duncan's resolve in that cause should ponder the National Education Association's July 4 call, at its convention in Denver, for his resignation.

The American Federation of Teachers also was highly critical of the secretary Monday at its convention in Los Angeles, though stopping short of demanding that he quit.

Still, the AFT did condemn Mr. Duncan for aligning himself "with those who have undermined public education, with those who have attacked educators who dedicate their lives to working with children, and with those who have worked to divide parents and teachers" by promoting "deprofessionalization, privatization and test obsession."

The AFT, like the NEA before it, expressed particular outrage over Mr. Duncan's praise for last month's state court ruling overturning teacher tenure in California.

Yet that case, Vergara v. California ruling, was widely and rightly hailed - and not just by conservatives - as an overdue step toward holding teachers responsible for the quality of their work.

As Secretary Duncan put it: "For students in California and every other state, equal opportunities for learning must include the equal opportunity to be taught by a great teacher. The students who brought this lawsuit are, unfortunately, just nine out of millions of young people in America who are disadvantaged by laws, practices and systems that fail to identify and support our best teachers and match them with our neediest students."

And: "Together, we must work to increase public confidence in public education."

Unfortunately, though, teacher unions have repeatedly lowered public confidence in public education by obstructing attempts to strengthen teacher accountability - and school choice.

That doesn't mean teacher merit pay should be solely based on students' academic progress. Such a program must fairly factor in the challenges faced by teachers in classrooms where too many children are unprepared, unmotivated and lack sufficient support at home for the educational process.

However, just as there has to be some reasonable way to measure the academic progress of students through testing (but not over-testing), there has to be some reasonable way to measure the performance of teachers.

The AFT says it has an "improvement plan" for Secretary Duncan, including "remedial" instruction that should set him straight. But in this case, the teacher unions are the truly slow learners.

That's because Mr. Duncan is rightly trying to teach the AFT and NEA that educational accountability and school choice are ideas whose time has come.

And the example of a Democratic secretary of education championing those positive causes should help make that lesson clear.