THE AIR BAG has become the center, oddly, of a profoundly ideological quarrel. That's the inflatable bag under a car's dashboard, to protect people in the front seat in a crash. Or, to be more specific, to protect those people who won't fasten their seat belts, and who will cut belts that fasten automatically. There is the issue: how far should the federal government go to preserve the safety of people who willfully refuse to preserve it for themselves?

The case for the air bag is that it saves lives, and it's worth saving the lives even of those careless and perverse souls -- representing perhaps four-fifths of the population -- who won't use their seat belts. Automobile manufacturers don't much like the bags, fearing that customers will resent them. Advocates of the bags believe that, without legislation to force them, the manufacturers will make the bags in such small numbers that the costs will be unreasonably high and the bags will shortly disappear altogether.

Legislation is now in the final stage of passage, in a congressional conference. It will say that, beginning with the small cars in 1983, cars sold in this country will have to provide "passive restraints" to protect front-seat passengers. An air bag is a passive restraint, but the manufacturers generally seem to prefer seat belts that lock themselves. Safety specialists expect that, people being people, many will tear out the belt systems. An air bag is less obtrusive and therefore less vulnerable to the owner's baser impulses.

The conference has agreed that the law ought to ensure enough air bags in use to provide a reasonable statistical base for measuring its benefits. But at this point an essentially good-hearted idea begins to turn into classically bad legislation.

Some manufacturers will have to provide air bags as options on some cars -- but which? The conferees have written the requirement to include as many of the foreign manufacturers as possible but not Chrysler or American Motors, which are deemed incapable of bearing the emotional strain. The bill is now supported by a strange alliance of those people who want air bags and those who want to harass Japanese imports.

The law will require each of the included manufacturers to provide the bags as an option on one line of cars. What's a line? The conference is now hung up on that final question. The narrow-liners say that it's a model -- for example, the Chevrolet Citation. The broad-liners say that it's all the cars built on a similar platform, or chassis -- in the same example, not only the Citation but also the Pontiac Phoenix, the Oldsmobile Omega and the Buick Skylark.

The narrow-liners are right and the broad-liners are wrong. The government has a large responsibility to protect drivers from dangers that a layman can't assess -- whether a brake is safe, or what happens to the steering column in a crash. It has minimal responsibility to protect drivers from a risk that anyone can see and understand -- like the risk of refusing to use a seat belt. The new law will guarantee that customers who want air bags will be able to get them in some models. That's enough. Beyond that, the future of the air bag will have to depend on whether people actually want it.