Still, Milo Kirk of Dallas, lobbyist for Mothers Against Drunk Driving, called the law a ''step in the right direction'' and vowed to press for a full ''open-container'' law during the next legislative session. ''The people that live here now are disgusted with the old Texas macho image,'' she said.

Despite a tightening of laws in recent years, 15 states, including Texas, had no laws as of January 1987 against consuming alcoholic beverages at the wheel, although there are local ordinances against it in many cities in those states, according to the National Highway Traffic Safety Administration. Until the Texas law goes into effect, it will still be possible to drive the 3,000 or so miles from Key West, Fla., to northern New Mexico without putting down your drink.

The other states in which anyone in a vehicle can drink are: Alabama, Alaska, Connecticut, Delaware, Florida, Georgia, Indiana, Louisiana, Maine, Mississippi, Missouri, New Mexico, Tennessee and Wyoming. This represents a considerable decrease from 1985, when 26 states had no laws, the National Safety Council said.

Until now, the police in Texas could not arrest someone drinking at the wheel unless he showed signs of intoxication. The new bill provides a $200 misdemeanor fine if the officer sees the driver take a drink, so there are questions about whether it can be enforced. A more potent bill was opposed by the Wholesale Beer Distributors of Texas, which neither supported nor opposed the final, diluted bill. The Distilled Spirits Council supported the bill. Bill Called Best Possible

Nonetheless, Joel Brandenberger, a spokesman for the chief Senate sponsor of the bill, Bill Sarpalius, a Democrat who represents the Panhandle, said this was the strongest bill possible. ''Everybody would have loved to have seen liquor banned from automobiles entirely, but that might have taken another two or three sessions,'' he said.