Missouri's House has narrowly endorsed legislation prohibiting labor contracts from requiring workers to pay union fees as a condition of employment.
The House voted 78-68 on Wednesday to give first-round approval to the measure, known to supporters as right to work. While there were enough votes to endorse the bill, the roll call was four votes shy of the number needed to send the legislation to the Senate on a separate vote.
Supporters say the measure protects individual rights of workers. Opponents contend it limits the ability of unions to bargain collectively for wages and benefits.
Democratic Governor Jay Nixon's opposition to the bill led Republicans to add a referendum clause that would send the issue to Missouri voters in August.