Missouri lawmakers this week easily passed legislation that would make it the 26th "right-to-work" state.
But the vote totals, if they hold, would be insufficient to override an expected veto from Democratic Gov. Jay Nixon.
Republicans hold veto-proof majorities in the state's Assembly and Senate, but enough GOP members broke ranks to take supporters under the two-thirds threshold needed to thwart a veto.
Right-to-work prohibits requiring employees to join a union or pay dues as a condition of employment. Supporters characterize the issue as a matter of worker freedom and believe it makes states more competitive; critics counter it helps depress wages, particularly for middle-class families.
In the Assembly, the right-to-work bill passed 92-66. Twenty-three Republicans voted against the proposal.
One day earlier, supporters in the state Senate used a procedural move to cut off a lengthy Democratic filibuster of the bill. The legislation passed 21-13 with four Republicans in opposition.
Senate backers would need two additional votes to get to the 23 required to override a veto, while the Assembly would need to get to 109 votes.
State Rep. John McCaherty, one of the Republicans to oppose the proposal, argued the bill would inject the state into the affairs of private businesses and unions.
"When you knock on their door next time, don't tell them that you kept government out of their life," McCaherty said.
Nixon, meanwhile, issued a veto threat on Wednesday.
"Attacking workers and weakening the middle class will not create jobs,” the governor said in a statement.