They wore deck shoes and sport coats, or dresses that came straight from work. And while there was more gray hair than long hair, the mood at last night's party honoring Ralph Nader was clearly '60s idealism.

"I first heard him speak in 1968," Dave Zwick, director of Nader's clean water action project, recalled. He was part of a crowd sipping Heinekens and munching on tortellini, salmon and potato pa te' at the Library of Congress that included Capitol Hill staffers, a few congressmen and scores of consumer advocates who in earlier times had been known as "Nader's Raiders."

"He said, 'There will someday be a public-interest movement . . . and just a few people working hard could make things happen.' I went up and said, 'Sign me up, Ralph.' "

The party, which attracted many of the original Raiders, the young crusaders who helped their leader launch the consumer movement, celebrated the 20th anniversary of the publication of Nader's landmark book, "Unsafe at Any Speed," an expose' of auto-safety problems.

Nader, his tie slightly askew, stood by sheepishly as Sen. Howard Metzenbaum (D-Ohio) declared him "a man who has truly made a difference in this country" and credited him for everything from auto recalls to bans on carcinogens to formation of the Environmental Protection Agency. Nader shook Metzenbaum's hand politely and stepped back nervously.

"If anybody's ever seen Ralph Nader uncomfortable, it's right now," quipped Joan Claybrook, the affair's organizer.

Nader was not shy, however, when it was his turn to address the crowd of more than 150: He jabbed the Reagan administration for what he saw as a failure to enforce public interest and environmental laws.

"The post-Reagan period is approaching fast, but not fast enough," he said. "There are reasons why Mr. Reagan is not winning all his battles and a lot of them are here tonight," he said, noting, as many had throughout the evening, that Nader alumni are now working throughout the country, inside government as well as outside.

Rep. Henry Waxman (D-Calif.), dubbed by partygoers as the "Consumer Congressman," Sid Wolfe, consumer author and head of Nader's Health Research Group, and journalist I.F. Stone were among the guests.

At least a few who remembered Nader gatherings that drew smaller crowds thought the affair was relatively fancy. "Am I at the right party?" former lobbyist Andy Feinstein asked. "This is far too upper crust from my memory of Ralph."

Claybrook, who now heads Rader's umbrella group, Public Citizen, joked that this party for Nader was "the first one I didn't have to ask people to pay to come to, and I didn't have to cook."

The group presented Nader with the back hood of a gold Corvair, the car that was the target of his famous expose'.