When a Virginia Republican legislator stood up at a recent Charlottesville fund-raiser to introduce his party's candidate for state attorney general, he came quickly to the point.

W.R. (Buster) O'Brien "knows more about criminal law in his little finger than his opponent knows in all five of her ringless fingers," said state Del. George P. Beard Jr. of Culpeper.

To some who heard the remark it was obvious that Democrat Mary Sue Terry's legal knowledge was not the issue; the issue was the fact that Terry is single.

In this fall's Virginia elections, Republicans have given a new -- and sometimes biting -- twist to the "family values" issue that long has been a staple of many political campaigns. Democrats in the state are complaining the GOP is using the issue to make unfair and often crude charges against their candidates, especially Terry.

"Some of the comments have been repugnant for Virginia politics," said Jay Shropshire, clerk of the Virginia Senate and a prominent Democrat. "They've unsuccessfully, by inference, tried to attack her . . . . Just because someone's single, should that preempt him or her from running for public office?"

Republicans say they were embarrassed by the legislator's remark and have not encouraged their supporters to such extremes. Family issues are a key plank in the state GOP campaign and the candidates will continue to stress them, because, as Republican spokesman Don Harrison said: "It's a way of saying what your root values are. People identify with that."

Last week when President Reagan appeared with the GOP ticket in Arlington, the family of gubernatorial candidate Wyatt B. Durrette was at center stage and one of his teen-aged daughters lifted her 6-week-old brother (the Durrettes' seventh child) above her head, displaying the infant before a packed ballroom.

Minutes later, Reagan wrapped his endorsement of Durrette around the candidate's image as a family man. "I guess one of the things I like best about Wyatt is that he has common sense enough to know the value of values," said the president. "Family, faith, freedom and opportunity aren't just campaign slogans for him, they're the foundation upon which his political philosophy is built."

It is a campaign issue that Reagan has parlayed into a strong vote-getter for both himself and others. Repeating the theme, Reagan told the Northern Virginia gathering: "I often think the real heroes of today are the parents, trying to raise their children in an environment that seems to have grown more and more hostile to family life."

"They must have polls that show those things are subliminal and effective," said state Sen. Joseph V. Gartlan Jr., a Fairfax County Democrat. "When you really think of it, does it tell you a lot about a candidate that he has a healthy, bouncing 6-week-old baby? I'm not sure it's considered a requirement for being governor of Virginia."

Even so, Virginia Democrats have used the issue this fall, although perhaps not as aggressively as the Republicans. The party's nominees, including Gerald L. Baliles, Durrette's opponent, have been quick to parade their families before the public.

To make their family points -- and their counterpoints -- the candidates of both parties in each of the statewide races have taken to the campaign trail with their wives, children, cousins and parents. Relatives and family members are often on podiums, in parades and in television commercials.

State Sen. L. Douglas Wilder of Richmond, the Democratic nominee for lieutenant governor, frequently points to his three children and notes that one is now enrolled at the University of Virginia law school. Wilder is divorced, as is his opponent, state Sen. John H. Chichester of Stafford County, who has remarried and campaigns with wife and 17-year-old daughter by his first marriage.

Unmarried candidates -- both male and female -- frequently are at a disadvantage against a married opponent, according to Larry J. Sabato, a University of Virginia political science professor who recently made an academic study of unmarried politicians. Sabato said that despite growing numbers of persons who choose to remain single, society still places a premium on the traditional family.

"Politicians are symbols," said Sabato, who is single. "They're supposed to be able to represent our values and ideas. And there's still a stigma in our society to being single. It means you've been unsuccessful, there's something the matter with you, you're gay or you're frigid."

Republicans say that family values are important to many Virginians. "People are interested in that," said Debbie Durrette, 22, the oldest of the candidate's seven children and the only one who campaigns for him full time. "The question I always get is 'How's your dad as a father?' The relationship he has with us and Mom. People look at that."

Some Democrats say the GOP candidates are using the family unfairly. "The Republicans are trying to use it against the Democrats who are pro-ERA and pro-choice," said state Sen. Clive L. DuVal, a Fairfax Democrat. He says that in the case of Democratic attorney general candidate Terry, the Republicans have more direct intentions. "They're also trying to point to specific situations where there is not a family."

Of the three Republicans seeking statewide office, O'Brien has been the most obvious in pointing to his family and his role as a husband and father of three children.

"It's apparent in our race, there's an attempt by Buster to try to get the voters to focus on things other than the candidate's qualifications," said John Jameson, a spokesman for the Terry campaign.

O'Brien denies that he is attempting to highlight the fact that Terry is single. "I think my life has been changed in the last 11 years during his marriage ," he said in an interview. "I've gotten a different perspective on life than when I was single. I'm not saying my opponent is not concerned."

Terry says she does not believe that her marital status should be an issue, but she nevertheless conspicuously portrays herself as part of a farmland family. One of her campaign television commercials features Terry at a family picnic table where she is cutting food for one of her young nieces.

When she announced her candidacy on the dock of her family mill and feedstore in tiny Critz, Terry told reporters: "So much of what I am is wrapped up in this farm. So much of who I am is my family."

At this past summer's party convention, Terry was nominated for attorney general by her father, Nathan, while her mother, sisters and other relatives surrounded her on the stage.

It prompted Speaker of the House A.L. Philpott to shout above a cheering crowd: "You want family to be an issue? This is just a small portion of what we can bring out of the hills and hollows of Patrick County."

Republicans acknowledge the family issue is a sensitive one in Terry race. Legislator Beard, whose remarks about Terry's ringless fingers angered Democrats, said: "I was trying to be funny. I wished I'd never said it."

Some in Virginia have had difficulty accepting a woman on the state ticket. The conservative Richmond News Leader, for example, posed what many saw as a slap at Terry in the spring.

Noting that Wilder had referred to the party's ticket as a "triumvirate," an editorial suggested the word referred to three men and asked: " . . . Have we missed something, and is the Democrats' designated candidate for Attorney General really -- to borrow from Johnny Cash -- a boy named Sue?"