A measure to roll back part of a contentious education reform in Mexico is on hold after the country's Senate rejected legislation previously passed in the lower house.

The bill represents a key campaign promise from President Andrés Manuel López Obrador. His allies enjoy majorities in both bodies, but the measure failed late Tuesday after a number of senators from his party were not present.

The measure now goes back to the lower house.

The original reform was passed in 2013 under previous President Enrique Peña Nieto and was promoted as an attempt to modernize education in Mexico. It imposed exams and evaluations for teachers and stripped unions of their longtime influence over hiring, salaries and promotions of teachers.

Critics said it failed to account for regional and historic inequalities.