As she worked the halls of Congress a couple of weeks ago, intruding on Senators, Candy Lightner allowed herself a flash of ebullience. "You know," she said, "I've already even picked out something to wear when the Senate votes on the bill. It's red. Red is for courage."

The founding mother of Mothers Against Drunk Drivers (MADD) was as bright as a Christmas tree last Tuesday, beaming from the gallery as the Senate passed the so-called "21" amendment by a vote of 81-16. (Although the amendment was to go back to the House for another vote, final approval seemed assured.) With "21," which will penalize states refusing to raise the drinking age to 21 by denying them federal highway funds, Lightner won the greatest victory of her four-year crusade against drunks behind the wheel. When MADD began, teenage drivers were involved in one out of every five fatal accidents, and even now nearly half of all highway fatalities involving 18 to 20-year-olds are alcohol related. Over the past decade an estimated 50,000 teenagers have died in drunk-driving accidents. Currently 27 states permit drinking under the age of 21. Drinking-age differentials between adjacent states, moreover, create disastrous "blood borders": Young drivers cross state lines to buy drinks, and those who get drunk run into trouble on the way home. As Lightner figures it, "21" will save an estimated 1,250 lives in a year.

The Senate vote marks an extraordinary triumph for an ordinary citizen. Lightner's endeavors have changed the nation's attitudes toward the long-ignored hazard of drunk driving. "As much as I complain about the system in this country," she says, "I've learned that it works. It allows grass roots organizations like ours to change the laws."

Since 1980, when her 13-year-old daughter, Cari, was killed by an intoxicated driver, Lightner, now 38, has been crisscrossing the country, cajoling state legislatures, making the case for stiffer penalties and contending with the powerful liquor lobby. The problem of teenage drinking, she would explain, dates from the Vietnam era, when the age of majority was rolled back from 21 on the notion that if you were old enough to fight, you were old enough to drink. Lightner considers this reasoning fallacious. "There are age limits," she would say, "for different levels of responsibility: 35 for President, 30 for Senator, 23 for FBI agent. We just don't feel that kids at 18, many of whom are just learning to drive, should be allowed to drink too."

Lightner and MADD, which she established in May 1980 in the den of her Fair Oaks, Calif, home, had their first Congressional success in October 1982. A bill introduced by Rep. Michael Barnes (D.-Md.) and James Howard (D.-N.J.) provided, among other measures, monetary incentives for states to raise the drinking age to 21. But the incentives didn't work. "Since last fall 23 states have considered raising the age to 21," says Candy, "but only four have actually boosted the age. We have been fighting the states' liquor lobbies."

In December 1983 a presidential commission, formed at the urging of MADD and several Congressmen, recommended federal penalties for states that did not enact a minimum drinking age of 21. Taking her cue from the commission, Lightner pressed for a new bill presented by Rep. Howard calling for the withholding of an escalating percentage of federal highway monies from states that did not adopt the "21" line. The House passed the measure enthusiastically on June 7. Remembers Candy, "There were Congressmen standing three and four deep on the floor speaking on behalf of the measure."

In the Senate Frank Lautenberg (D.-N.J.) offered an identical amendment, and on June 13 President Reagan boarded the "21" bandwagon. Yet Lightner had to deal with last-minute dissent in the Senate. Moving with the aplomb of a seasoned pol, she arrived at a compromise with Lowell Weicker (R.-Conn.), who felt that her measure discriminated against teenagers; she agreed to attach incentives for states to toughen sentencing laws. Lightner could not, however, convince a small minority who argued that the amendment was a violation of states' rights.

Withal, the attractive divorcée, who used to sell real estate in California, has become so polished, so adept at public speaking, that some people have suggested she has a political future. But Lightner says, "There are too many other things I want to do with MADD." Even with "21" apparently in the bank, she feels that MADD, with its 291 chapters in 44 states and half-million supporters, is just starting to change public attitudes. "I think people are more aware now, they're more conscious," she says. "We're seeing Fortune 500 companies providing taxi service for employees who've had too much to drink at office parties. We're seeing the National Restaurant Association recommend the elimination of Happy Hours that offer two drinks for the price of one. We're seeing car dealers against drunk drivers, truckers against drunk drivers. We're seeing police set up sobriety checkpoints. All of these measures would have been unpopular four years ago."

Serena Lightner, Cari's 17-year-old twin sister, says that MADD has become her mother's life. Serena stayed behind in California to finish high school this year when Candy moved with son Travis, 13, from Fair Oaks to Hurst, Texas, where MADD relocated its national headquarters. Occasionally Serena takes time from her job as a restaurant hostess to campaign for MADD. Candy admits to pride in her organization's accomplishments, but little pleasure. "What bothers me," she says, "is that all this came about because of children who have died.

"Yes," she continues, "I have thought about Cari from time to time during all of this. In fact the other day when we were trying to go through the Senate parliamentary procedures, I remember saying, 'Okay, honey, you've come this far with us....' People may think that's weird, talking to a child who's dead. But I've always had a feeling that Cari was present in some way, along with all the other children who've been killed. There's a constituency up in heaven, I suppose, rooting for us."