Lawmakers in two Democratic-controlled states are considering limits on self-checkout stations claiming they bring with them safety concerns — but their objections appear to be focused on protecting union jobs.

Self-checkouts have grown in use and popularity over the past few decades. A recent survey found that 77 percent of shoppers prefer self-checkout, finding they are faster than staffed checkout lines.

Unions are pushing back, claiming the machines are creating understaffing and unsafe conditions in stores.

In Washington, House Bill 1739 would only allow self-checkouts in supermarkets when regular checkout lanes are also available for use. It would also require one employee to be dedicated to supervise every two self-checkout stations.

The bill would further limit self-checkout use to people with 15 items or less.

State representative Mary Fosse, who previously worked in the grocery industry, claims that self-checkout stations are a misguided cost-cutting measure.

Ms. Fosse says grocery workers face “unchecked automation and chronic understaffing” that is dangerous. “We know it can be a convenience but when it’s used as a way to eliminate workers and cut labor costs, that’s when it puts workers and customers at risk,” Ms. Fosse says.

Proponents say workers are bearing the brunt of customer anger over malfunctioning machines and confrontations with shoplifters.

At a hearing on the proposal, grocery store worker James Reed testified that they create unsafe conditions for workers, noting that he often manages up to 12 registers. “I am alone, juggling what should be multiple positions. It’s not just stressful, it’s dangerous,” Mr. Reed said. He added that when machines malfunction they curse him out and hit the machines.

Mr. Reed said a woman threw a steak at him one day because she was upset it didn’t ring up as a buy-one-get-one deal on the self-scanner.

“People deserve to feel safe and respected at work,” Mr. Reed said. “My job has turned into one of stress, fear, and frustration.”

Stores that break the rules would face a $100 a day civil penalty up to $10,000 for multiple violations.

Representative Joel McEntire questioned the need for the government oversight. “Wouldn’t it be best to have that discussion in the public sector, where the customer would be able to make their decision with their dollar?” Mr. McEntire asked.

Washington Food Industry Association representative Katie Beeson also spoke out against the bill. She said requiring one worker for every two self-checkout lanes would be “overkill” and defeat the purpose of having them.

The California legislature is also considering regulating self-checkout. A bill reintroduced this session would require minimum staffing at stores that use self-service checkout. It would also require a clerk staff a manual checkout if self-checkout stations are open. It would also limit the number of items at self-checkout stands to 15.

The bill would further require that any store that wants to add self-checkout stations to notify workers at least 60 days in advance with fines of $1,000 per violation per day.

Unions in California are backing the measure.

“As self-checkout machines have proliferated through grocery and retail locations in every state, so have the problems,” representative of the United Food and Commercial Workers union, Kristin Heidelbach, testified at a recent hearing. She claims that workers having to manage multiple self checkout stations is leading to a burnt out workforce.

Proponents also claim that increased theft can be tied to self-checkout and ends up costing more than the savings on fewer employees. One study they cite showed self-checkout machines caused 16 times more shrink than checkout via a cashier.

Some large retailers like Walmart, Target, and Dollar General are scaling back use of self checkout because of rising retail theft but critics of the legislation say that the companies were able to make those decisions on their own and didn’t need government intervention.