As local public school teachers await word on whether city officials will approve another round of seniority-based raises, here is an intriguing and growingly open secret about teachers unions: they increasingly are the object of disdain among some of their rank and file.

Denver is a good place to start. When teachers there sought greater control in such areas as work day structure, hiring and pay, they found themselves at loggerheads not with school officials but union bosses. The teachers eventually won out. In Los Angeles, there have been similar clashes as teachers seek to shed bureaucratic union bonds.

Critics of unions generally and teachers unions specifically frequently have cited as problematic collective bargaining's practice of negotiating pay based on time served rather than performance. More than a third of teachers leave the profession in their first five years over pay concerns, which some studies insist is based in part on the fact that private-sector jobs offer the chance to earn merit raises.

Unions long have contested this claim, but now they must do so against hard evidence. More than 170 New York City schools are participating in a pilot performance pay program. It turns out that not everyone favors the idea of advancement by mere prolonged existence.

Better pay for the best teachers likely would keep more of them in the classroom. But unions fight such initiatives with the spirit of beleaguered 19th century factory workers taking on Pinkerton thugs. Why- Because allowing individual workers to earn more based on their own merits would give them greater autonomy and, presumably, weaken unions as a result. The issue is not about fair pay but control.

Many teachers know this, but feel caught in an elaborate, paralyzing snare.

Teachers unions represent one of the country's most powerful political forces. More than 400 delegates at the 2004 Democratic convention were members of either the National Education Association or the American Federation of Teachers, according to the Lexington Institute. Only California among the states had a larger delegation. Understandably, few teachers are willing to openly challenge organizations with such clout.

Still, we hope the example in New York City will inspire more teachers to press for what so many of them want - pay that rewards them based on how effectively they perform rather than on their simply logging in the years.

Union leaders, meanwhile, would do well to look to history, which shows that their power is far from assured. Inflexibility on issues such as merit pay could well be to modern teachers unions what years of rigidity on pay and vacation were to the steel unions that dominated labor into the 1970s: the seeds of a descent into irrelevance.