The California legislature has given the Reagan administration a setback in its campaign to promote mandatory seatbelt laws, which some critics charge is being waged to eliminate a federal rule requiring passive restraint devices in new cars.

At issue is a July 1984 Department of Transportation regulation that could force state legislatures to choose between passing laws requiring the use of the seat belts or accepting federally imposed automatic-crash-protection rules.

California lawmakers, unhappy with DOT's proposition, over the weekend passed a "sunset" seat belt bill that shifts the burden of choice -- and its attendant political liability -- back to the Reagan administration.

Earlier versions of the state seaat blt bill were supported by California Gov. George Deukmejian. He is likely to sign the legislature's version, but even if he vetoes it, anti-DOT temperament in the legislature is strong enough to back an override, some state legislative sources said.

The California law, the first of its kind in the yearlong seat belt campaign, raises the ante in a lobbying war to which the nation's auto makers have made a commitment of $15 million a year through 1989.

Critics in the insurance industry and elsewhere charge that the auto companies' real aim is to eliminate federal rules that would require the installation of automatic crash protection devices such as air bags in all passenger cars sold in the United States.

Representatives of auto makers deny that allegation. They say that their primary purpose in supporting mandatory-use laws is an immediate and more practical improvement in passenger safety.

But if other states follow California's lead, either by adopting new seat belt laws or by amending those already on the books, DOT could be forced to require automatic crash protection, the agency's critics say.

At the very least, DOT would be forced to accept full responsibility for resolving the issue, according to critics.

The 1984 DOT regulation says that automatic crash protection devices must be available in cars sold in the United States beginning Sept. 1, 1986.

But the rule has a big escape hatch that the auto companies, represented by Detroit-based Traffic Safety Now Inc., are trying to keep open with their lobbying campaign.

The escape provision says that the automatic crash protection rule would be rescinded if states with two-thirds of the nation's population pass laws by April 1, 1989, that require their residents to use seat belts when they drive.

California has about 10 percent of the nation's population and would be an important ally for those who favor mandatory-use laws over automatic crash protection.

Some state legislatures have tried to thwart the escape plan by approving mandatory-use laws that, by DOT standards, apparently are too weak to scrap automatic crash protection. The California bill is such a measure, but it goes one step further. It contains a "sunset" clause that says the state's mandatory-use law would expire if DOT attempts to use it to void the federal requirement for automatic crash protection.

The California "sunset" law thus eliminates the concern of some that DOT -- which so far has not ruled on the validity of any of the 15 state seat belt laws passed to date -- would certify weak laws to scuttle the automatic-crash-protection rule.

"The California law is important because it absolutely precludes that important state from being used to rescind the crash-protection standard," said Jack Quinn, an attorney representing State Farm Insurance Co. Air bags and similar devices, as well as seat belt laws, are needed to protect the motoring public, Quinn said.

Auto industry officials generally decried the "sunset" provision, but said that they would try to work with the California law anyway.

Administration officials declined comment on allegations that DOT might accept inferior seat belt laws. But Diane K. Steed, head of the National Highway Traffic Safety Administration, said she was "glad to see that California has joined" other states in putting a seat belt law on the books. "While each law enacted to date is unique, in some respects, all share the common purpose of saving lives and reducing the number of serious injuries" on the nation's highways, Steed said.