A new and very small literature has provided evidence that students have lower test scores in larger school districts and in districts in which the district's teachers union has negotiated a contract that is more favorable to the district's teachers. The teachers’ unions at the state and national levels contribute a great deal of money to candidates for state and federal offices. This gives the unions some influence in passing (defeating) bills that would help (harm) the state's teachers. We introduce two novel measures of the strength of the state-wide teachers union: union dues per teacher and union expenditures per student. These reflect the key role of political influence for state-wide unions. We provide remarkably strong evidence that students in states with strong teachers unions have lower proficiency rates than students in states with weak state-wide teacher unions.