Proponents of Proposition 45, the ballot measure that would give the insurance commissioner the power to reject unreasonable health rate increases, say they’re simply trying to do for health premiums what was done for auto and home insurance more than 25 years ago.

Those who oppose Prop. 45 argue the measure puts too much power in the commissioner’s hands and it could destabilize the state’s barely 1-year-old Covered California program, the online marketplace created by the federal health law to offer private insurance to residents.

Voters in 1987 passed Proposition 103, which gave the insurance commissioner regulatory powers over auto, home and other policies, a move that has saved consumers tens of billions of dollars over the years.

Supporters, including state Insurance Commissioner Dave Jones, hope Prop. 45 will give health insurance policyholders the same benefit.

“This is about stopping excess health insurance rates. There’s no agency in the state of California that has the power to reject them,” said Jamie Court, president of Consumer Watchdog, the Santa Monica group behind both propositions.

With the federal health care law now requiring most Americans to have health insurance or face a financial penalty, Prop. 45 supporters say it is essential that the state’s elected insurance commissioner have the authority to make sure those rates are affordable. With health insurance rates rising five times the rate of inflation over the past decade, consumers need protection, they argue.

But opponents argue that if proposed increases are allowed to be challenged, negotiations between Covered California and health insurers over the costs of policies to individuals and small businesses could be undermined. An extended challenge could upend Covered California’s three-month enrollment period or federal subsidies available to more than 80 percent of those who buy through the exchange could be delayed or decreased.

“The time required … would completely disrupt the ability of Covered California as a health insurance store to provide consumers with prices in time to begin shopping on Nov. 15,” said Jon Kingsdale, the former head of Massachusetts’ health insurance exchange and now a health care consultant. Kingsdale was commissioned by the No-on-Prop. 45 campaign to write a report about the measure.

Supporters of the measure dispute that argument, pointing out that 35 states already have the power to regulate health insurance premiums — and that five of them have exchanges that are structured very similarly to Covered California’s.

The Prop. 45 campaign has split loyalists of the federal health law, putting some in the same camp with the major health insurers, while others are siding with the backers.

Anthony Wright, executive director of Health Access, a statewide consumer advocacy coalition, supports the federal health law as well as Prop. 45. He said efforts built into the health law have helped curb costs but don’t go far enough.

“Many of the arguments made by the opposition are unfounded. The legitimate issues that have been raised are resolvable if people work together,” he said, referring to the timing concern and other possible disruptions.

Opposition comes mainly from the state’s four largest health insurers — Kaiser, Blue Shield of California, Anthem Blue Cross (and its parent company WellPoint) and Health Net — but also includes hospitals, doctors and some labor and small business groups.

More than $37 million has been contributed primarily by the insurers to defeat the proposition compared with Prop. 45’s $1.8 million in contributions, $1 million of which came from the California Nurses Association to fund the campaign’s first commercials. The commercials, which started airing last week, feature nurses urging voter support.

No-on-Prop. 45 commercials have been flooding the airways for weeks with opponents, doctors groups among them, discrediting the notion of taking power away from an “independent commission,” meaning Covered California, and giving it to a politician, in this case, the state insurance commissioner.

For people like Juanita Hagemann, an individual policyholder who has endured rate increases, Prop. 45 offers an opportunity to stop insurers from “behaving badly.”

“We need some oversight in California to guard against the manipulation I believe happens, especially with individual policyholders,” said Hagemann, 63, a self-employed bookkeeper who lives in Mill Valley.

John Walkmeyer of San Ramon said that before the federal health law went into effect, he would have supported giving the commissioner power to set rates.

“Now that Covered California has responsibility for negotiating features and pricing of plans purchased through the exchange, I think the insurance commissioner’s role should be limited to jawboning,” said Walkmeyer, 68. “I'm opposed to having too many 'deciders’ involved.”

Victoria Colliver is a San Francisco Chronicle staff writer. E-mail: [email protected] Twitter: @vcolliver