For years they were the one-two punch of the consumer movement, he blasting away at the corporate robber barons and she adroitly lobbying on Capitol Hill.

It ended yesterday with the finality of two Corvairs totaling each other - head on.

Ralph Nader demanded that his close friend and erstwhile chief lobbyist, Joan Claybrook, resign as head of the National Highway Traffic Safety Administration.

In a word - actually, in an 11-page single-spaced "Dear Joan Claybrook" letter - Nader said she has been, in her eight-month tenure, a failure.

Just as resolutely, Claybrook responded that she has no intention of quitting. She said she promised President Carter she would serve four years and that she will honor the pledge.

What's more, Claybrook added in a brief statement handed to reporters, she is proud of her accomplishments at the NHTSA, although she genially conceded "we still have a long way to go."

And in a subtle dig at her old crony. Claybrook said she is "fully aware" of the complexities of running a regulatory agency, an experience Nader has not yet had - from the inside, at least.

Transportation Secretary Brock Adams, her boss, quickly spoke up in support of Claybrook, calling her "the kind of person who gets things done."

Then, maybe echoing the consumers who looked to Nader and Claybrook as their gurus, Adams said he thought it "sad that two of the leading safety advocates in the country have to be split by this kind of thing."

Before Claybrook, a 40-year-old Baltimore native, was nominated by the President in March. She was director of Congress Watch, a Nader organization that lobbies and monitors Capitol Hill.

She was one of Nader's closet personal friends, and shared with him the policy and personnel decisions affecting the Nader public-interest groups.

In his lengthy, itemized indictment throughly as tough as any of the dozens he has launched against corporate captains and bureaucrats. Nader charged that Claybrook has "etched a trail of averted or broken promises" in highway safety.

He said that she took the job full of idealism and knowledge about "the neglected potential of the 1966 auto safety law" but that she had fallen far short of meeting her promise to turn NHTSA into a model agency.

Nader listed nine of Claybrook's previous positions, each a tough stance on different facets of highway safety and then compared them with her performance.

"This is more than a failure of leadership: it is a failure of nerve," Nader said. The issue, he added, is not that specific goals have not been met but that "nothing has been initiated toward so many of those goals."

Nader's chief complaint was that Claybrook had disagreed philosophically, yet publicly supported Secretary Adams' decision this year to allow the auto industry to delay a phase in of passive safety restraints, such as air bags, until the 1982 model year.

"You did not believe in this decision. Yet, you supported and defended it against your well-known conviction that a requirement for across-the-board 1981 model year was ample lead time for even the footdragging of the auto companies," Nader wrote.

"Through a profound mismanagement of NHTSA's role in the decision, you undermined the regulatory integerity of your own agency," he added.

Claybrook's brief, written statement did not respond to specifics of the indictment. But she said the Carter administration "has done more than any other in advancing the cause of highway safety."

Among Nader's other complaints:

The agency has done nothing to fill the gaps in safety standards for vans and light trucks, of which more than 3 million are sold annually, despite Claybrook's belief in such standards.

Claybrook apparently has changed positions on the need for "dynamic test requirements" for safety belts. He said she is now "negative" on the issue.

She has failed to live up to her belief that tough, competent advocates should be recruited into the agency - advocates who would have the fortitude to criticize their superiors when necessary.

"Resignation, accompanied with full explanation and revelation, is now your most constructive course of action . . . Returning to the citizen movement will permit you to have far greater impact . . . than your current incarceration of spirit and behavior," Nader concluded.

Yesterday afternoon Nader showed up uninvited at an impromptu meeting Claybrook held with reporters. He listened to her read her response, and then, with a smile, asked a technical question. She answered.

Then the bureaucrat under attack went to a meeting. Ralph Nader conferred with reporters in the hall.