One of the few issues in education that divides Democratic front-runners Hillary Clinton and Barack Obama in this presidential campaign is merit pay.

Yesterday, Sen. Clinton criticized the idea during a campaign stop at an Iowa elementary school. While the senator from New York said she supports the less controversial idea of incentives for teachers who work in high-need areas, Clinton, who won the endorsement of the American Federation of Teachers in October, denounced the idea of merit pay as “demeaning and discouraging.” (However, she does like the idea of school uniforms for students.)

Teachers’ unions usually fiercely oppose the idea of merit pay, although some states, such as Texas and Minnesota, and districts, such as Denver Public Schools, have tried it with mixed success. Both the AFT and the National Education Association are fighting off any merit-pay ideas that may crop up during the No Child Left Behind reauthorization.

It was a politically daring Sen. Obama of Illinois who told the NEA at its July national convention that merit pay was worth pursuing. However, he softened the idea a bit by pledging to work with teachers to craft such a system and to base merit pay not just on an “arbitrary test score.”

Obama is talking about merit pay in the larger context of a very tough issue: closing the achievement gap among minority and non-minority students. But the issue of merit pay has a group of formidable foes, so Clinton can continue to harp on this difference between her and one of her chief opponents, and shore up her support among public school teachers and union backers.

Update: Read more about Obama’s education plan, and his ideas on teacher compensation, in my newer blog item here.