Unions are outspending opponents in a bid to defeat a right-to-work law in Missouri ahead of a referendum that will serve as a barometer of popular support for organized labor.

A union-backed group in the state has spent $15.2 million ahead of Tuesday’s vote to undo the law passed by the Republican-led legislature in 2017, according to the latest filings with the Missouri Ethics Commission. That is nearly five times the $3.2 million spent by two groups that support the law, which allows private-sector workers to opt out of paying union dues or fees.

The fight in Missouri has risen to prominence following a June Supreme Court ruling that struck down mandatory union fees for public-sector workers. That decision already is cutting into union coffers. If Missouri adopts right-to-work, it would be another blow for unions.

If the ballot initiative succeeds, it would be the first time a right-to-work law is overturned by popular vote, according the AFL-CIO, the nation’s largest federation of labor unions.

Indiana is the only state ever to flip, according to the AFL-CIO. Legislators there overturned a right-to-work law in 1965, but the state eventually passed another one in 2012.