Missouri voters, through a labor-organized veto referendum on Tuesday, overwhelmingly rejected the state's "right-to-work" law passed by the Republican-controlled legislature last year, which bans compulsory union fees in all private-sector workplaces.

Why it matters: This is a major victory for unions, which have spent millions on a campaign to defeat the law that would have diminished their effectiveness. Missouri has become the latest battleground over union fees following the Supreme Court’s conservative-majority decision in June that public-sector unions cannot mandate mandatory fees from non-members who refuse to join.

The details: The law was defeated by a 2-to-1 margin. The vote followed a referendum filed by unions last year to overturn the policy that already existed in 27 other states.

• The highly partisan fight over union fees has been growing in other states. Since 2012, Indiana, Kentucky, Michigan, Wisconsin and West Virginia have become right-to-work states.

Supporters of the policy said it would strengthen states economically and gave workers a choice on whether to join unions.

The other side: Union leaders argue that its part of a concerted effort by Republicans to weakened their power because labor groups have traditionally supported and provided money to Democratic candidates in crucial states.

• The fees collected can only be used for collective bargaining, not for political purpose. Unions argue that nonmembers who pay fees benefit from contract negotiations, salary and time off, so everyone should make a financial contribution to prevent freeloading.

What they're saying: Richard Trumka, president of AFL-CIO, the nation’s largest federation of labor unions, said in a statement: "Missouri is the latest sign of a true groundswell, and working people are just getting started. The defeat of this poisonous anti-worker legislation is a victory for all workers across the country."