Because of an editing error, a quotation in a story yesterday on Americans for Democratic Action was incorrectly attributed to Rep. Barney Frank (D-Mass.) His sister, Ann Lewis, the group's national director, said, "There is an internal pendulum in American politics, a self-correcting mechanism. When you go too far in one direction, you have to go back the other way. It is at least arguable that the conservative peak has passed.

In President Reagan's Washington, the nation's oldest and largest liberal organization is trying to stage a comeback.

At a time when many former liberals are calling themselves "neo-liberals" or even "moderates," the true believers of Americans for Democratic Action are hoping that conservatism is on the wane as part of an inevitable pendulum swing in American politics.

"The public is a lot more liberal than it knows," said ADA's president, Rep. Barney Frank (D-Mass.).

In an effort to help the public discover this fact, the ADA has launched efforts to attract members, increase its clout and do a better job of spreading liberal ideology.

Under new national director Ann Lewis the group, founded in 1947 by such eminent liberals as Eleanor Roosevelt, Hubert H. Humphrey and labor leader Walter Reuther -- and all but moribund in the age of Reagan -- recently began to "prospect" for new members through mass mailings.

It now hosts seminars for summer interns on liberal issues that range from Nicaragua to civil rights. It has begun a fellowship program and has set up an educational fund to do research. This past weekend it held its first policy conference, designed to reexamine liberal positions and strategies.

The moves, Frank said, are intended to put the liberal movement back on the political landscape.

"ADA, for most of its life, has been a major player in the Washington political game and not as much in the grass roots," he said. "We now realize we have to work in the country, building membership and having events in liberal communities."

"Economics is the key to this agenda," Lewis told the conference group yesterday. "By our silence we have allowed the right to restructure the public debate. We have to advocate economic growth with equity."

Lewis, formerly political director of the Democratic National Committee, said the ADA is attempting to double its membership in two years.

Lewis said ADA's heyday, in terms of members, probably was the early 1970s, at the height of the antiwar movement. Since then its numbers have dwindled to a mailing list of about 85,000 and a paid membership of about half that.

The moves toward younger voters, through the fellowships and intern seminars, were spurred by the realization that liberals no longer could rely on the youth vote, she said.

"We went for too long under the assumption that anyone under 21 was actually a liberal," Lewis said. The 1984 election, when younger voters were among Reagan's strongest supporters, disabused the ADA of that idea.

"There are liberal student activists out there, and we've seen them," Lewis said. "Too often, they've felt isolated; the conservatives have networks."

Recently a group of liberal college graduates, all under age 30, organized a youth chapter of ADA, known as the New American Agenda. The event pleased many in the ADA, which Lewis said has many members who politically came of age with and knew Eleanor Roosevelt.

The policy conference last weekend was designed to reexamine liberal positions and find more effective methods for spreading the word.

Lewis said it is time to figure out "how to speak less to ourselves and more to everyone else. Too much time has been spent in perfecting each other, instead of explaining where we are."

Frank told the group at its opening session: "I don't think the problem is the substance. We have to show how to advance our goals in an appealing way."

The ADA, he said, "is in a position, more than in the last few years, to demonstrate that principled and thoughtful and responsible liberalism is not only morally correct, but politically correct as well."

"There is an internal pendulum in American politics, a self-correcting mechanism," he added. "When you go too far in one direction you have to go back the other way. It is at least arguable that the conservative peak has past."