Iowa Republicans passed legislation Monday preventing cities and counties from raising the minimum wage ― and negating hikes that already went into effect in four counties.

Such “pre-emption” laws have become all the rage in GOP-controlled states, and the Iowa measure now heads to the desk of Republican Gov. Terry Branstad. He is expected to sign it.

Under the law, localities could not implement minimum wages higher than the state level, which is currently as low as allowed under federal law: $7.25 per hour. They also could not pass their own paid leave measures requiring employers to give workers a certain number of sick days per year.

Pre-emption laws have become a popular way for state legislators to take legal autonomy away from local jurisdictions when it comes to the minimum wage and paid leave. But advocacy groups for low-wage workers said the Iowa measure is unusual in that it would work retroactively.

“The bill’s passage marks the first time anywhere in the U.S. that state lawmakers have actually taken away raises from workers who already received them,” Christine Owens, director of the National Employment Law Project, said in a statement. Owens called the legislation “callous” and “a new low.”

The bill’s passage marks the first time anywhere in the U.S. that state lawmakers have actually taken away raises from workers who already received them. Christine Owens, director of the National Employment Law Project

If it’s signed, the higher local minimum wages currently in effect would have to revert to $7.25, lowering pay for many workers. The measure could set off a legal battle, as similar laws have elsewhere.

By one analysis, nearly 30,000 workers were affected this year when Johnson, Linn, Wapello and Polk counties raised their local wage floors to surpass $10 per hour by 2019. That number would have grown significantly as the wage floors continued to increase in coming years.

Backers of the pre-emption bills say they want to avoid a “patchwork” of different minimum wages within their states. Republicans in Iowa said blocking local hikes and paid leave proposals would give employers more predictability when it comes to labor costs, according to the Des Moines Register. The legislation was supported by the state’s grocer and restaurant associations, as well as the alliance of local chambers of commerce.

Twenty-three other states currently have pre-emption laws, according to NELP. Some of those laws had been on the books for years, but the majority of them were only passed recently, as progressive campaigns such as the Fight for $15 saw tremendous success in pushing local minimum wage proposals. While the federal rate of $7.25 has remained stagnant since 2009, a majority of states now have higher minimum wages than the federal level, and many cities and counties have gone as high as $15.

Democrats and labor unions say the pre-emption laws are undermining local, democratically made policy decisions and hurting the working poor. The most controversial pre-emption law was passed in Alabama, with the explicit purpose of blocking a minimum wage hike in the city of Birmingham. The city itself is roughly three-quarters African-American, with 30 percent of residents living below the federal poverty line. The Republican legislature that passed the pre-emption bill is primarily white. Minimum wage backers there have filed a federal civil rights lawsuit against the state.