In San Francisco, since the passage of Proposition 13, the property tax burden has shifted from commercial to residential property owners. In 1975, commercial property owners paid 59 percent of property taxes. Today, they pay 43 percent, according to Phil Ting, San Francisco’s assessor-recorder. That’s true even, though San Francisco’s population has remained relatively flat, and the city has seen a downtown building boom.

Under Proposition 13, taxes are reassessed when a property changes ownership. Residential properties tend to do so more frequently than commercial ones. “The bottom line is that residential properties trade about every seven years, and commercial properties trade significantly less often,” Mr. Ting said in a telephone interview. “Even when they do trade, thanks to Prop. 13, they don’t always great reassessed.”

A new bill from Assemblyman Tom Ammiano seeks to close a loophole that lets some commercial property owners avoid such reassessments — and, perhaps, to find a weakness in Proposition 13’s seemingly impregnable political armor.

Under current rules, a change of ownership of a business does not prompt a reassessment of property taxes unless greater than 50 percent of the business is purchased by a single buyer. So if three buyers each acquire a third of a business, property taxes stay frozen at the previous owners’ level.

While a majority of commercial properties are reassessed when sold, some have completely changed hands without property taxes going up under the current rules.

Under Mr. Ammiano’s bill, an assessment would occur regardless of how many owners buy the property. The bill passed out of the Assembly’s revenue and taxation committee on Tuesday with a few amendments that narrowed it. The legislation is now on its way to the appropriations committee, where it will be heard in late May.

“This is about fairness,” said Mr. Ting, who supports Mr. Ammiano’s bill. “When you have new owners purchase a home, they get reassessed. It’s not fair when you have new owners purchase a commercial property, and they don’t get reassessed.”

Some critics say that Sacramento should not fiddle with Proposition 13. “”We see it as an attack on Proposition 13 protections,” said Kyla Christoffersen, a policy advocate for the California Chamber of Commerce. “We think that the existing law is very effective.” She added, “we believe that this primary impact of this bill will be to increase property taxes of small business owners.”

Others argue that California’s economy is so shaky right now that businesses cannot afford additional taxes.

“Any increase in property taxes that is foisted against business is going to have a negative impact on the state’s economy,” said David Wolfe, legislative director for the Howard Jarvis Taxpayers Association, which is named for Prop. 13’s author. “When you have a 12.6 percent unemployment rate, and with 2.3 million Californians out of work, this is the wrong time to continue to be passing taxes against businesses.”

Despite the association of the measure with tax relief for homeowners, commercial interests were originally among its most important backers. As the author Daniel A. Smith pointed out in his 1998 book, “Tax Crusaders and the Politics of Direct Democracy,” Howard Jarvis was a lobbyist for the Los Angeles Apartment Owners Association.

“Vested commercial property owners — particularly apartment owners and realtors — were the key to the organizational and financial success of Jarvis’s initiative,” Mr. Smith wrote.

Supporters of the bill contend that the property tax burden in California has dramatically shifted from commercial to residential property owners since 1978.

“What’s amazing about it is that it’s so consistent in almost every county,” said Lenny Goldberg, executive director of the California Tax Reform Association, the co-author of a new report on the issue. In 55 of the 58 counties in California, commercial property owners now pay a smaller percentage of those taxes than they did before Proposition 13, according to the report. “For businesses, the number of ways of avoiding reassessment are legion,” Mr. Goldberg said.

Ms. Christoffersen takes issue with that report’s methodology, saying that it classifies many properties, like apartment buildings, as residential, which she contends should really be considered commercial properties.

“Commercial properties contribute about two-thirds of the overall property tax revenue for the state, as was the case before Proposition 13 was passed,” she said.

While the California Tax Reform Association is a sponsor of Mr. Ammiano’s bill, the organization’s executive director noted that the bill would only do so much. “It’s a step toward improving the law; it’s not a solution,” Mr. Goldberg said. He would like to see a constitutional change that would require commercial properties to be periodically reassessed at market value, which is the practice in 49 other states.