The way things are going for unions, Republicans on Capitol Hill won’t even need to bother passing a national right-to-work law to decimate organized labor. The states are taking care of that all on their own.

On Monday, Missouri Gov. Eric Greitens (R) signed a right-to-work bill that was sent to him by the state’s GOP-controlled legislature. That makes Missouri the 28th right-to-work state and the sixth state to pass such legislation since 2012.

Right-to-work laws give workers the option to stop supporting unions while still enjoying the benefits of union representation. Unions strongly oppose them because they tend to decrease membership and weaken the labor movement.

Although they’ve been around for decades, such laws were confined mostly to the South and West until recently. They’ve become increasingly popular in recent years as Republicans have taken over state legislatures and governors’ mansions and pursued business-friendly reforms. The laws benefit Republicans politically since they weaken a major pillar of the Democratic Party.

In some cases, unions have been steamrolled as soon as state Democrats lost power. In Kentucky last month, Republicans rushed through right-to-work legislation as the first major order of business for the year, after Democrats lost the state House for the first time in almost a century.

The only thing holding back right-to-work in Missouri last year was the veto of then-Gov. Jay Nixon, a Democrat. After Greitens defeated Democrat Chris Koster in November, it became clear that Missouri would be added to the growing list of right-to-work states, which now includes several in the traditionally union-dense Midwest.

Workers who aren’t in unions often don’t understand how right-to-work laws function. Under U.S. labor law, a union must represent all the employees in a workplace it has unionized, even those who may not want to be in a union. Unions argue that it’s only fair for all workers to contribute to cover the costs of bargaining, and they prefer to lock in contracts that stipulate as much.

But right-to-work laws make such arrangements illegal. That, in turn, allows workers to opt out of paying fees to a union that will have to represent them anyway ― a situation unions deride as “free riding.” Supporters of right-to-work laws argue that no worker should be required to support a union, regardless of whether it bargains for them.

The groups that push right-to-work laws have had stunning success recently. And the more states that fall in their column, the more likely others become to follow suit: Many Republicans claim they support right-to-work laws because they want their states to be as attractive to businesses as their neighbors.

Republicans in Washington introduced a federal right-to-work bill last week, which President Donald Trump has suggested he might support. If it became law, that measure would make the entire country right-to-work, superseding the 22 states that currently have no such law. But although Republicans control both chambers of Congress, that bill appears unlikely to survive a Democratic filibuster, at least for now.