Labor unions across the country are paying big money to defend forced unionism, pouring millions of dollars into a Missouri ballot initiative to overturn the state's right-to-work law.

An AFL-CIO-affiliated campaign called We Are Missouri has raised more than $17 million since putting the state's 2017 right-to-work law on the ballot. That is more than triple the $5 million output of the bill's supporters, according to election tracker Ballotpedia. The campaign has attracted plenty of outside investment, including the Teamsters and Carpenter national unions, which have combined for nearly $2 million. The gap has grown even larger in terms of spending with We Are Missouri already dropping $16.3 million on the referendum, nearly four times as much as the $4.6 million spent by supporters.

We Are Missouri did not respond to request for comment.

The campaign has made Missouri ground zero in the right-to-work debate. Unions have traditionally turned to the courts to fight laws that ban mandatory union fees. Those suits have failed in both state and federal courts across the board. Missouri's labor powers opted to bypass a judicially focused approach and instead appeal directly to voters. The strategy may prove a wise one, as state polling has found heavy support for unions. A July Missouri Times poll found right to work losing 56-38 despite the Republican leanings of the 1,000 people surveyed. The same poll gave GOP Senate nominee Josh Hawley a 48-46 lead over incumbent Democrat Sen. Claire McCaskill and found 50-44 favorability rating for President Trump—well ahead of the RealClearPolitics average of 42-54.

Union spending may help drive those numbers as Missouri residents respond to an aggressive courtship campaign, which has included large ad spending, expansive get-out-the-vote ground operations, and numerous public rallies. Labor watchdogs have attempted to counter the spending mismatch with ads of their own. The National Right to Work Committee has spent $1 million in television ads, including a recent 30-second spot with former St. Louis Police Officer Association president Sgt. Gary Wiegert (retired).

"I believe that a union or any other organization should not force anyone to join them," Wiegert told the Washington Free Beacon. "Oftentimes these [referendums] come down to who spends the most money, the fact that they mobilized their forces and spent this much money should tell you something."

Wiegert's ad prompted a cease-and-desist letter from the association, which opposes right to work. The union took issue with Wiegert wearing an SLPOA polo shirt in the ad and emphasized that Wiegert was expelled from its ranks in 2012 for unauthorized lobbying—Wiegert has represented pro-life groups and other conservative organizations at the state capitol in the past. The letter accused the committee of "using our copyrighted logo without permission to intentionally mislead Missourians." While Wiegert pulled the ad down from his Facebook page, the National Right to Work Committee said it will continue to promote the ad in the days leading up to the election.

"As of now the plan is full steam ahead on the ads despite the cease-and-desist letter," a spokesman told the Washington Free Beacon. "Obviously when necessary, we’ll look at any legal options."

Some labor watchdogs said the outpouring of union support is not surprising. Since 2011, right to work has been adopted in traditional pro-union states across the Rust Belt and coal country after the rise of Republican state legislatures, from West Virginia and Kentucky, to Wisconsin, Michigan, and Indiana. If Missouri’s voters reject right to work it would deliver labor a major victory. Vinnie Vernuccio, a senior fellow at the Mackinac Center for Public Policy, said the outpouring of financial support demonstrates the stakes for organized labor: "It is sad but not surprising that unions are spending millions of dollars to have the ability to get workers fired for not paying them."

Right-to-work supporters attempted to push back on the referendum by proposing an amendment to the state constitution, which would give residents another chance to vote on right to work in November. The House voted 93-54 to approve the amendment, but the proposal failed to get on the ballot after the Republican-controlled Senate did not hold a vote on the issue. The failed bid for a constitutional amendment makes the August referendum the deciding vote on whether Missouri remains the 28th right-to-work state.

The referendum will be held Aug. 7.