JEFFERSON CITY, Mo. (AP) — The steady march of new right-to-work laws in Republican-led states hit a wall in Missouri, where voters resoundingly rejected a measure that could have weakened union finances after national and local labor groups poured millions of dollars into the campaign against it.

Missouri’s law against compulsory union fees was defeated Tuesday by a 2-to-1 margin, nearly a year after the measure adopted by the state’s Republican governor and Legislature had been scheduled to take effect. It was put hold after unions successfully petitioned to force a public referendum.

The election results effectively vetoed the Missouri measure and halted a string of stinging losses for organized labor. Since 2012, five other once historically strong union states had adopted right-to-work laws as Republicans gained strength in state capitols, raising the total to 27 states with such laws.

The Missouri referendum marked the first chance for voters to weigh in on union powers since the U.S. Supreme Court ruled in late June that public sector employees cannot be compelled to pay fees to unions. Missouri’s ballot measure essentially would have extended that to all private sector employees in the state.

Missouri voters had last rejected right to work in 1978, when national union membership was more than double its current rate of 10.7 percent.