The California Chamber of Commerce and other groups representing employers are starting to line up to oppose the initiative to legalize marijuana, charging that Proposition 19 would allow pot smokers to light up on the job and operate dangerous equipment while stoned.

Stepping up the campaign on Thursday, the chamber released a five-page analysis that starts: “Imagine a workplace where employees show up to work high on marijuana and there is nothing you can do about it.”

Allan Zaremberg, the chamber’s chief executive, criticized the measure, saying it would set a high bar for employers to act against workers using marijuana by requiring proof that they were actually impaired. “Prop. 19 creates a new protected class of California citizens,” he said.

The initiative’s proponents dismissed those claims. “It’s a lie that’s designed to raise money from California employers and other hot-button organizations,” said Dan Rush, a union official working for the campaign.

Only a few sentences in the initiative refer to the workplace, but they are likely to inspire contentious debate. In addition to the chamber, the Assn. of California School Administrators and the League of California Cities have opposed the initiative for similar reasons. The chamber voted to oppose it months ago, but chamber officials said questions from its 15,000 member businesses led them to ask Jennifer Shaw, an employment lawyer, to take a closer look.

“Prop. 19 is going to add to employers’ burdens,” Shaw said, suggesting that it could make workplaces less safe, increase liability insurance costs and encourage worker lawsuits.

The initiative would prevent employers from firing or disciplining workers who use marijuana unless it “actually impairs job performance.” Laura Preston, legislative advocate for the school administrators, said officials could not stop a bus driver suspected of using pot until after an incident. “You can see the headlines,” she said. “School principal lets bus driver drive while high.”

David Rosenfeld, a union lawyer with ties to the Proposition 19 campaign, said employers are riled up because they could not simply fire employees who test positive for marijuana, which can stay in the body for days, but would have to show that their work suffered. “There are lots of people out there who use marijuana responsibly,” he said, “and it doesn’t impact their work.”

In her analysis, Shaw concluded that employers would not be able to ban toking at work. Rush noted the initiative bars the use of marijuana “in public or in a public place.” But Shaw said it’s not clear workplaces are public places, citing a recent court decision that concluded grocery stores were not.

Zaremberg also said businesses would not be able to comply with the Drug-Free Workplace Act, a requirement for receiving federal money. Analysts for the Assn. of California School Administrators and the League of California Cities agreed. The federal law requires employers to ensure that controlled substances, including marijuana, are not used in the workplace.

If being drug-free is a job requirement, Rush said, then employers would have the right to prohibit employees from using marijuana. In that case, he said pot- smoking employees would meet the criteria of being impaired because they would fail to meet the job qualifications.

Preston, who estimated that $9.4 billion in federal dollars comes to California’s public schools and colleges, acknowledged that the issue was complicated, but said it is not worth the risk to see who is right.

john.hoeffel@latimes.com