Each state sets its own laws regarding teachers unions, and the laws vary widely, not only on whether teachers may be forced to pay union dues. They vary on whether and on what issues a union may collectively bargain and whether unions may call strikes.

Nearly half of states — 24 — are so-called right-to-work states, in that state law prohibits requiring employees to pay dues to a union they do not otherwise support, according to the Department of Labor and the National Conference of State Legislatures. Although federal law governs union elections and other factors, the issue of “fair share” or “agency” fees has remained the purview of states.

State legislators have introduced 63 bills in 28 states this year concerning union dues, according to the NCSL. This is up dramatically from 2012, when bills were debated in 19 states and passed in four, and 2013, when 23 states debated bills but only one passed. Some of the laws created new “right-to-work” laws, while others added enforcement or notice provisions to existing laws.

The Thomas B. Fordham Institute, a conservative education think tank, in a 2012 report ranked the strength of teachers unions in the 50 states and District of Columbia, as measured by membership and financial resources, political involvement, permissible bargaining items, state policies and perceived influence. All 10 of those ranked strongest required agency fees. The 10 ranked weakest — all in the South, plus Arizona — prohibit agency fees.