Except for the gray at the temples and an occasional smile, Ralph Nader looks much as he did 20 years ago when he burst onto the national scene talking about the buyer's right to a safe car.

Back then, testifying before Congress about what was wrong with American automobiles, Nader wore a white shirt, narrow tie and dark rumpled suit. The hair was close-cropped and the look was lean. The style was that of a humorless and relentless advocate.

This past week many of the same somber trademarks were evident as Nader appeared at the annual meeting of the Consumer Federation of America after an absence of six years and used the occasion to urge a more aggressive advocacy.

Unlike the leaders of some other movements, who have been eclipsed by younger, more radical followers, Nader has managed to stay in the vanguard of the consumer movement.

"There are different perspectives on some issues," said CFA president Stephen Brobeck, "but his speech stressing the need for grass-root consumer support demonstrates once again that strategically he is one step ahead."

Consumer activist Jack Gillis said that Nader's accepting the CFA invitation to speak was an indication that he "may have mellowed just a little bit." Another clue, he said, was the shadow of a smile on Nader's face when the audience applauded. "The old Ralph never would have smiled," Gillis said.

Nader also appears to be "a little more tolerant now," according to Gene Kimmelman, who worked closely to Nader at Congress Watch before joining CFA last year. "Nader is not trying to control everyone and slap their wrists if they don't do what he does."

But the Nader intensity still burns.

Hunched over the speaker's podium, with scribbled notes as a guide, the man who has launched a thousand consumer causes spoke for nearly an hour.

"It's time for consumer groups to stop being taken for granted, to stop being nice guys, and to start addressing the big issues," he exhorted.

He complained about the lack of vigor in today's consumer movement. He vilified public of- ficials who have opposed his consumer objectives. He challenged his listeners to join him in working for a better consumer society.

"Listening to him, I was inspired again -- it kind of got my juices flowing and I remembered why I got into the consumer movement to begin with," Gillis said. "It also occurred to me that it is possible for one man to make a difference."

Those close to Nader, who will be 51 this month, say that he is as active now in pressing for consumer reforms as ever. He still travels hundreds of miles a year to meet with small groups of consumers, encourage them and spur them into action. He still makes more than $100,000 a year and gives most of it away to public interest causes. He still comes out regularly with critical reports compiled by aides who work long hours for low wages.

After the 1980 election of President Reagan, Nader dropped out of the Washington limelight. At the same time, his relationship with other consumer movement leaders soured as they sought ways to continue in the new more conservative environment.

Nader has used the time since then to try and build a network of decentralized local consumer groups around the nation.

"His theory is that the consumer movement shouldn't rely on government and shouldn't be trying to get into government," said Kimmelman, who is credited with bringing Nader and CFA back together. "He thinks that the consumer movement needs its own base . . . and he thinks that the base should be local consumer groups, like CUB (Citizen Utility Board) and Buyers Up."

The CUB groups, which now operate in Wisconsin, Illinois and Oregon, represent consumers in utility rate matters. Buyers Up is a Nader program that helps Washington area subscribers buy heating oil at a discount price.

In addition to starting new groups, Nader believes that existing consumer-oriented organizations, such as the American Association for Retired Persons, the American Automobile Association and the credit unions, should take a more activist position on behalf of consumers.

That conviction triggered some of the most biting remarks in Nader's speech to the consumer assembly.

"When he sees groups like AARP and AAA and credit unions not fighting and not using their base for activism, and therefore compromising consumer interests, he is willing to lash out across the board," Kimmelman said.

"And I would say that he can be pretty stinging."