A split is growing between cities that want to require private companies to give workers paid sick days and states that are determined to stop them.

In the last three years, a dozen states have banned localities from passing paid leave requirements, more than doubling to 22 the states that now outlaw such local ordinances. The push for so-called pre-emption laws is backed by the Koch brothers and the American Legislative Exchange Council, a membership organization of state legislators who favor limited government.

The state moves come in response to the increasing number of cities and counties passing paid sick days ordinances. Since 2015, more than 20 cities, as well as eight states, have approved measures mandating that companies provide local workers with paid sick leave. Since San Francisco approved the first paid sick leave ordinance in 2006, paid sick day requirements have been passed in 35 cities or counties and 11 states.

Backers of required sick leave say they’re giving an essential health benefit to workers — one that will improve public health by keeping ill employees at home.

Opponents say paid sick leave will cost employers too much, and that a patchwork of conflicting local and state policies will only cause confusion.

“There’s a real pitched battle going on in a lot of places right now between cities that have decided that they really want to protect workers’ rights and workers’ health, and state legislatures that don’t want to interfere with businesses at all,” said Sherry Leiwant, co-founder and co-president of A Better Balance, a New York-based group that supports paid leave.

The United States is the only developed country without a national paid leave law, says the International Labour Organization, a United Nations agency. Nearly a third of all workers in the United States don’t have paid sick days, according to the American Journal of Orthopsychiatry.

Yet bipartisan support for the benefits is higher than ever. Ninety-four percent of Democratic voters and nearly 80 percent of Republican voters favor paid sick leave laws, according to a 2015 New York Times-CBS News poll.

A handful of states that prohibit local sick leave ordinances, including Maryland, Oregon, Rhode Island and New Jersey, do require businesses to provide sick leave, but bar cities from going beyond the state requirements. Other states, such as Wisconsin, don’t have a state rule and prohibit cities from passing their own.

In 2008, voters in Milwaukee approved a paid sick leave measure with support from nearly 70 percent of voters, making the city the third in the country, behind San Francisco and Washington, D.C., to approve one.

But in 2011, newly elected Republican Gov. Scott Walker and the GOP-led Legislature reversed the Milwaukee measure and approved a law to pre-empt other Wisconsin cities from following its lead.

The clash over paid sick leave is part of a broader divide between conservative states and their more liberal cities on a wide range of issues, including minimum wage laws, fracking, plastic bag bans, municipal broadband and anti-discrimination ordinances that protect lesbian, gay, bisexual and transgender residents.