Teacher evaluation has already been a sticky issue in the debate over reauthorization of the Elementary and Secondary Education Act. And now it’s getting even stickier.

Three moderate senators—Mary Landrieu, a Democrat from Louisiana; Scott Brown, a Republican from Massachusetts; and Joseph Lieberman, an independent from Connecticut—are not happy with recent changes scaling back the teacher evaluation provisions in a bill sponsored by Sens. Tom Harkin, D-Iowa, the chairman of the Senate Health, Education, Labor, and Pensions Committee, and Michael B. Enzi, the top Republican on the panel.

Those changes won the bill bipartisan support from Republicans who weren’t fans of the idea of mandating evaluations. And it pleased the teachers’ unions.

The trio wrote a letter urging Harkin and Enzi to reconsider recent changes to the ESEA bill in the area of teacher evaluation. If you’ll remember, the original version of the bill called for all states to develop evaluation plans. But now, only districts that get Teacher Incentive Fund grants (which are doled out to schools that want to create alternative pay programs) will have to create the programs.

That’s a big step backward, the senators say. They call the new language “a disservice to our nation’s students.”

But one of the Republicans who pushed for the change, Sen. Lamar Alexander, R-Tenn., a former secretary of education, explained the reasons behind the change in a speech on the Senate floor Tuesday.

The new version of the bill “does not include an order from Washington” to evaluate teachers, he said. But he said it would allow states to use federal funds to help create evaluation systems, which would help states figure out the best way to evaluate educators.