When a new group called The First Committee threw a voter registration dance at the Wax Museum nightclub in Southwest Washington recently, the crowd that showed up was just the kind the organizers were hoping to attract: a veritable fashion fair of impeccably dressed, young, black professionals, each one a profile in middle-class achievement.

Surveying the crowd of about 2,000, which included consultants, salesmen and saleswomen, a few doctors and lawyers and other white-collar, briefcase-toting types, group chairman Hank Wilson was impressed. But when he took to the stage to explain the reasons for a dance whose theme was voter registration, he indicated he also was worried.

"Right now we are safe and warm, with nice clothes, but that's no reason to feel permanently secure," Wilson, a 36-year-old media consultant, told the group. "We are learning fast that politicians can change our lives and take us to the poorhouse."

In recent years, young middle-class blacks generally have been financially secure and politically apathetic.

Now, however, economic conditions and President Reagan's policies have combined to threaten the government jobs and affirmative actions programs by which many in this category attained their middle-class status.

The First Committee is a new group on the city's political landscape. Its impact and staying power remain to be seen, but it is representative of what some see as a larger trend toward greater political activism among the young black middle class.

"In traveling around the country, one of the signs that the economic conditions are having an adverse impact on the black middle class is that members of that class are starting to get organized and show up at rallies and political forums that, two years ago, only poor folk used to be involved in," said Clarence Mitchell III, a real estate salesman and Maryland state senator who is not connected with The First Committee but whose business puts him in touch with a cross-section of the area's black professionals.

"During the Kennedy-Johnson years, the black middle class was only rising and was still identified with the low-income black community," Mitchell said. "Then we got into the 1970s and the black middle class began to divorce itself from the poor. This has changed in the 1980s, with Reagan becoming one of the greatest unifying factors of the black community."

Mitchell echoed the findings of a recent National Urban League study, which concluded that throughout the country blacks see the economic gains of the Great Society years slipping away.

"The extremely conservative political climate and record-breaking recession that this country is now enduring . . . seriously threaten the existence of an emerging, still fragile, black middle class," said the report, entitled, "A Dream Denied: The Black Family in the '80s."

The First Committee was formed last March and is headed by a 50-man steering committee made up of Washingtonians previously best known for the lawn parties they held. The group now boasts of 1,300 members regularly enlisted in what has been the organization's main activity so far: voter registration.

"We're not just concerned about the middle-income group, but for the grassroots, too, because many of us have been there," said psychiatrist Kenneth Smothers, a member of the committee. "We are aware that during times like these we can easily slip back into the bottom as the middle gets wiped out."

The idea for The First Committee came about during a party late last year. Discussion among Wilson and others turned to how economic pressures were affecting friends and acquaintances. Some were losing their homes to foreclosures, others having their cars repossessed.

"Once, we were doing all right," said Wilson. "Then we moved into the '80s and the houses and luxury apartments got a little cooler and the soft places got a little harder and the jobs got a little shaky and one day the problems came tumbling down, slapping us into reality. It occurred to many of us that if you don't vote, you won't be heard."

During the past few months the committee has been attempting to establish itself as a legitimate bloc of voters. Each of the Democratic candidates for mayor has attended the group's political forums. Last week, after the group endorsed Marion Barry for mayor, two of its members received appointments to city advisory boards.

"They are very job-oriented and they want to know if there will be a place in the government for them," said City Council member Charlene Drew Jarvis (D-Ward 4), a mayoral candidate, after meeting with The First Committee. "Many of them are highly trained folk who have been shut out of the job market, and they want to know what can be done to help."

The group's membership offers numerous examples of people who considered themselves secure until economic conditions worsened and they found themselves out in the cold.

Barbara Sanders said she had been a floor manager at a local department store until she was laid off last week. "It's a racial and economic thing going on," she complained. "They are just trying to move black people out. They are trying to go back to the days when blacks just worked in the kitchen. It's times like these when black people really have to band together."

Vernon Preston said he was laid off from his job with the D.C. public schools. "Maybe if I had been involved earlier this wouldn't have happened," he said. "But you still have to keep fighting, and this is the best way I know -- get involved."

And member Robert Sewall gave this view of the group and its situation: "We're talking about a coalition of people stuck between getting tax breaks and getting welfare."

Looking out over the crowd that had gathered at the Wax Museum, as participants alternately listened to music and political speeches, Wilson shrugged approval.

"The idea is that first you have to get people out before you can talk to them about registering to vote," he said. "Some people say we are just a social thing, but that is part of the plan. We're saying politics shouldn't be dull. It should be fun, too. You really don't have anything to lose; then again, you may have everything to lose."