The state Senate is poised to approve sweeping changes to public schools that would prevent most third-graders who don’t read well from being promoted, end teacher tenure and give schools A through F letter grades based on performance.

The proposal won tentative approval 31-15 along party lines Thursday. Democrats argued that new limits on job security would make teachers’ jobs vulnerable to complaining parents and vindictive principals, and that the letter grades stigmatize rural schools as failures. Republicans urged “bold action” to improve academic performance, specifically, to have children reading at grade level by the time they enter fourth grade.

Republicans said the bill wasn’t perfect, but moves to remedy problems of weak young readers and bad teachers.