California lawmakers have advanced legislation that would be the strictest measure in the US limiting when police can use deadly force.

The bill would allow officers to use lethal force only when there is no reasonable alternative. Supporters say the measure would save lives by pushing officers to use de-escalation tactics, which could curb unnecessary killings that disproportionately affect black Americans.

Families of those killed by police gave emotional testimony in front of the state assembly’s public safety committee on Tuesday. The bill was inspired by last year’s police shooting of Stephon Clark, an unarmed black father of two shot dead by police in his grandparents’ back yard when officers mistook his cellphone for a gun.

In testimony, Ciara Hamilton described how Barstow police killed her cousin, Diante Yarber, last year after they said the car theft suspect hit two squad cars and nearly struck an officer. His family says the car was barely moving and that Yarber, who was black, might have lived had officers provided immediate medical attention.

“What does that tell us about policing in California and America? It’s that black and brown people are not safe from state-sanctioned violence,” Hamilton said.

Elizabeth Medrano Escobedo told lawmakers that Los Angeles officers could have used other tactics instead of killing her son, Christian Escobedo, last year. Police said he was sleeping behind a parked car with a loaded handgun.

“This bill can save mothers from grieving the loss of their children, which is what I’m experiencing right now,” Medrano said.

The Democratic assemblywoman Shirley Weber of San Diego said: “It’s time to make clear that the sanctity of human life is policing’s highest priority,” adding later that her proposal “is designed to change the culture of policing in California”.

California police kill people at a rate 37% higher than the national average per capita, according to the American Civil Liberties Union, which supports the legislation. Police in the state killed 172 people in 2017, more than two-thirds of whom were people of color. Police in Kern c

ounty, California, killed more people per capita than in any other American county in 2015, a Guardian investigation found.

Weber’s measure got party-line support. But it faces a tough fight in the full assembly.

Even some supporters on the public safety committee said it went too far and would require changes as lawmakers try to balance the safety of officers and those they are tasked with protecting.

The Democrat Reggie Jones-Sawyer of Los Angeles said a tougher standard would do little good without buy-in from law enforcement organizations.

The two Republicans on Tuesday’s panel opposed the measure they said could make officers hesitate for a fatal second if they have to consider alternatives to lethal force.

Law enforcement groups are supporting a radically different legislative plan to curb deadly shootings, which would require that every department have policies on when officers should use de-escalation tactics and other alternatives to deadly force.

The Sacramento county deputy sheriff, Julie Robertson, testified on Tuesday that her partner, Mark Stasyuk, died last fall during a gunfight and she hesitated as the suspect shot at her with only his back exposed.

“I recall in that moment thinking that if I were to shoot him in the back, I would be the next officer in the news being scrutinized for my actions,” Robertson said. “The thought of having to second-guess my actions in that moment is frightening ... This bill makes me wonder if sacrificing everything is worth it.”

The Plumas county sheriff’s deputy Ed Obayashi, a use-of-force expert, called Weber’s measure “an exercise in legal futility” because he predicts judges will interpret the language the same way they do court rulings. Prosecutors would have to prove an officer was criminally negligent, which carries a high legal burden.

Weber acknowledges officers would have to egregiously violate a policy to face charges but expects the standard would deter shootings.