Update, Wed 9/11, noon: The Assembly passed AB5 on a vote of 56-15. It now goes to Gov. Newsom.

SACRAMENTO — The California Senate on Tuesday passed gig-work legislation that could transform the state’s employment landscape by turning many independent contractors into employees. The vote was 29-11, along party lines.

The bill now heads to the state Assembly, where lawmakers approved an earlier version but must agree to amendments. If it passes a final vote there, AB5 goes to the desk of Gov. Gavin Newsom, who wrote a Labor Day op-ed supporting the measure.

Hundreds of thousands of independent contractors, including Uber and Lyft drivers; DoorDash, Uber Eats, Instacart and Postmates couriers; Amazon Flex drivers; taxi drivers; translators; medical professionals; franchise owners and more, could become employees after the law takes effect in 2020. Unions, which pushed passage of AB5, hope to organize newly minted employees, especially those at gig companies.

State Sen. Maria Elena Durazo, D-Los Angeles, who co-authored the bill, began Tuesday’s debate by blasting tech companies and some traditional employers, saying they have, for decades, exploited workers with false contractor labels.

“Let’s be clear, there’s nothing innovative about underpaying someone for their labor and basing an entire business model on misclassifying workers,” Elena Durazo said.

The law’s impact could be even more far-reaching. Amid concerns over income inequality, the issue of employment status has become part of the national conversation — and California often sets the pace for the rest of United States. Democratic presidential candidates including Bernie Sanders, Kamala Harris, Elizabeth Warren, Julian Castro and Pete Buttigieg support the bill. Buttigieg joined drivers last month at a pro-AB5 rally outside Uber’s Mid-Market headquarters. Sanders last year introduced similar legislation called the Workplace Democracy Act, with Warren and Harris among his co-sponsors, although it will not pass as long as the Senate remains under Republican control.

AB5 codifies and expands a groundbreaking California Supreme Court decision from last year known as Dynamex, which uses a simple three-part criteria, the ABC test, to determine employment status. It says a worker is an employee if the worker’s tasks are performed under a company’s control; those tasks are central to that company’s business; and the worker does not have an independent enterprise in that trade.

Proponents say that companies call workers independent contractors to avoid paying minimum wage, overtime, workers’ compensation, unemployment insurance and a range of other benefits that can add 30% to labor costs. Misclassification costs California some $8 billion a year because of lost wages, taxes and expenses, as well as subsidizing social safety-net assistance for the workers, said the bill’s author, Assembly member Lorena Gonzalez, D-San Diego.

But opponents, who include both companies and workers, say they value the flexibility of independent contractors. Businesses in several industries, including ride-hailing, gig deliveries, hospitals, newspapers and truck owner-operators, warned that adding the costs of employment could be devastating and result in higher charges to consumers and curtailed service.

On Tuesday evening, Gonzalez announced she had agreed to delay the bill’s implementation by one year for newspaper delivery drivers, following demands in the Senate.

“While I personally disagree with this delay, I’m willing to allow the newspaper industry the additional year to comply if it means those delivery drivers and nearly a million other misclassified workers are provided the minimum wage, benefits and workplace rights of Assembly Bill 5,” Gonzalez said in a statement.

Senators rejected 12 sets of Republican-sponsored amendments, which largely would have added carve-outs for more professions. During a debate that stretched into the night, GOP senators said the bill failed to add clarity to the Dynamex decision.

“It picks winners and losers when it should be providing a framework on how you are determined to be a common-law employee or an independent contractor,” said Sen. John Moorlach, R-Costa Mesa.

A lengthy list of professions won exemptions from AB5, largely because they typically set their own prices and negotiate directly with their customers. They include doctors, dentists, psychologists, insurance agents, stockbrokers, lawyers, accountants, engineers, direct sellers, real estate agents, hairstylists, commercial fishermen, travel agents and graphic designers.

Uber, Lyft and other gig companies negotiated unsuccessfully to be exempted. They now plan to take their case directly to voters with a ballot initiative in November 2020 seeking to create a new category of workers who would be independent but could receive some benefits and a guaranteed wage floor. Uber, Lyft and DoorDash have ponied up $90 million to support the measure.

Carolyn Said and Dustin Gardiner are San Francisco Chronicle staff writers. Email: csaid@sfchronicle.com, dustin.gardiner@sfchronicle.com Twitter: @csaid, @dustingardiner