An emotional, grass-roots movement to raise Virginia's drinking age, having won the support of Gov. Charles S. Robb, now appears strong enough to achieve its goal, some key legislators said yesterday.

During his televised "State of the Commonwealth" address Wednesday, Robb advocated raising the legal drinking age to 21, even though a commission he appointed last June to study drunk driving has not finished its work. Robb's endorsement sweetened the prospects of a campaign that had already become one of the most popular causes in this election-year legislative session.

"The age is definitely going to be changed," said Del. Warren E. Barry, a Fairfax County Republican who first sponsored a bill to raise the drinking age in 1976. "The question is by how much."

Virginia law currently forbids the sale of wine and hard liquor to anyone under 21, but allows 18-year-olds to drink beer in restaurants and bars and 19-year-olds to buy beer in stores. Although the session is only two days old, at least five separate bills have been introduced to raise the drinking age for beer either immediately or gradually over the next few years.

The intense lobbying has flooded some legislators with mail, as recently formed and competing organizations like Mothers Against Drunk Driving (MADD) and Many Against Drunk Driving (also known as MADD) disseminate stories of relatives killed in drunk driving crashes. Some legislators have promised to support the change even as they question whether the change, similar to a bill enacted in Maryland last year, will reduce drunk driving deaths significantly.

Even some proponents of tougher drunk driving laws say they fear raising the drinking age might cause youths to circumvent the law--by driving into the District, for example, where the drinking age remains 18. Others, including even one of the MADD groups, say it discriminates against responsible college students and young working people, who, as the popular argument held when the age was lowered in 1974, are old enough to vote and serve in the military.

"Sure, there's some underground opposition," said House Majority Leader Thomas W. Moss Jr., a Norfolk lawyer who has represented restaurants and bars before the state's Alcoholic Beverage Control Commission. "But they're quiet about it because it's like opposing God, motherhood and apple pie."

Moss, who survived a difficult primary last fall, said he would probably vote for the bill because "quite frankly, the political pressures are such that it would be hard to vote against."

Scores of legislators rushed from a noon session in the Capitol today to a luncheon meeting of the governor's task force on drunk driving, where Robb repeated his call to raise the drinking age.

"I know, of course, this proposal isn't going to be popular with everyone, including responsible youths and restaurant and tavern owners," Robb said. But Robb added that drunk driving deaths "are a tragedy we simply must deal with for the sake of our families and friends."

Robb ticked off statistics compiled by the state Department of Transportation showing that alcohol-related deaths among youths between the ages of 16 and 19 increased 1 1/2 times since 1975, the year after Virginia lowered the drinking age for beer from 21 to 18.

"We're all aware that the national consciousness has been aroused," said Robb, citing magazine cover stories and grass-roots lobbying.

Some Republican legislators suggested that the popularity of the cause may have been as persuasive to Robb as the statistics. Robb "jumped on the bandwagon and took a position that everybody knows is going to happen anyway," said Republican Sen. Wiley F. Mitchell Jr. of Alexandria. "After the whole legislature was charging up the hill, he jumped out and got in front of us."

Even some in Robb's own party questioned the wisdom of raising the drinking age. "I drank when I was 16 and got my driver's license and so did everyone in this room," said Del. Bernard S. Cohen, an Alexandria Democrat who said he is considering voting against raising the drinking age.

Other legislators said they feared a change in the law could foster youthful disrespect for the legal system and create enforcement problems, with young people getting beer from 21-year-old friends or from military bases, where the legal age will remain 18.

"They'll be driving in cars across the bridges and into Georgetown, just like they used to do before we lowered the drinking age," Barry said. But proponents argue that 16- and 17-year-olds will be less likely to come by alcohol illegally. "My argument has always been that we've got to keep it out of the hands of high school age children," Barry said.