WASHINGTON — A movement gaining momentum in Congress and some school systems across the nation would boost pay for exceptional teachers in high-poverty schools, a departure from salary schedules based on seniority and professional degrees that have kept pay in lockstep for decades.

Lawmakers are debating this month whether to authorize federal grants through a revision of the No Child Left Behind law for bonuses of as much as $12,500 a year for outstanding teachers in schools that serve low-income areas.

National teachers unions denounce the proposal for "performance pay," saying it would undermine their ability to negotiate contracts and would be based in part on what they consider an unfair and unreliable measure: student test scores.

Debate over the proposal has exposed unusual fissures between the influential unions and longtime Democratic allies. Some education experts say the unions are out of step with parents and voters who support the business-oriented idea of providing financial incentives for excellent work.

Rep. George Miller, D-Calif., chairman of the House Education and Labor Committee, said that the teaching workforce is leaking talent and that his proposal would help rejuvenate it. Young teachers watch their friends "go off and get paid for their time and ingenuity" in other fields, Miller said. "In teaching, you go as fast as the slowest person."

Miller's proposal — building on recent federal steps to encourage incentive pay— would provide grants to school systems that choose to pay bonuses to teachers who excel in high-poverty schools, worth up to $10,000 in most cases and $12,500 for specialists in math, science and other hard-to-staff subjects. Decisions on who gets extra pay would be based on student test gains and professional evaluations. Miller's aides said they had no cost estimate for the measure.

In the District of Columbia, a five-year, $14 million federal grant is fueling a pilot program to reward teachers and principals in a dozen high-poverty public schools each year that achieve the strongest gains in test scores and share successful strategies with others. Details are being worked out by the city school system, the local teachers union and a partner organization, New Leaders for New Schools.

The approach is also being tried in a dozen charter schools with help from a private grant. Charter schools are publicly funded but independently operated.

The D.C. Preparatory Academy charter school adopted another performance pay plan designed by the national foundation-funded Teacher Advancement Program. Its model pairs teacher evaluations with professional development and training.

One day last week, math teacher and mentor MaryKate Hughes observed how another math teacher set goals and expectations for his students. In another classroom, Hughes made notes on a science teacher's pacing and preparation. Newer teachers can receive bonuses of as much as $2,000 based on test score improvements and evaluations by master teachers and principals.

"Our goal is to find good teachers who can become great teachers," Hughes said.