The phrase that most people seem to remember from Jimmy Carter's farewell address is his warning against "single-issue groups and special interest organizations." The sentiment is widespread, and bipartisan. President Reagan said recently that he finds it "difficult not to call some of them 'selfish interest groups.'"

But when did all the interest groups became "special interests"? People who joined together to influence decisions in Washington used to be called "interest groups," a gray, neutral term that allowed the possibility that they have a legitimate role. Now the word "interest" never appears without the adjective "special" and its tinge of asking for more than one has a right to receive.

With all interest groups lumped together as "special interests," it becomes harder to separate those that try to elect or defeat candidates from those that lobby in a non-partisan way in Washington. We also blend groups that speak for a single cause or company with those that represent classes of individuals, or institutions or even elected officials. Nor do we distinguish groups that welcome -- and perhaps hasten -- the decline of political parties from those that work well with the parties and want them to be strong.

Some would argue that all interest groups are the same in one respect: they all believe their interest to be a vital part of the national interest, and act accordingly. That may be true, but it leads to another question: Do interest groups make it harder for the federal government to act in the national interest?

In the Federalist paper No. 10, James Madison saw the clash of interest groups as an integral part of the American system. "The regulation of these various and interfering interests forms the principal task of modern legislation," he wrote, "and involves the spirit of party and faction in the necessary and ordinary operations of government."

What Madison feared most was the ability of a majority or seeming majority -- led, we could imagine, by a new president flush with the power of electoral victory -- to define the national interest in any way it sees fit. A minority faction "may clog the administration [and] convulse the society," says the Federalist No. 10, but only a majority can "sacrifice to its ruling passion or interest both the public good and the rights of other citizens." That is why the country needs "the greater security afforded by a greater variety of parties."

Madison couldn't foresee precisely how interest groups would evolve over 200 years; he thought in terms of 18th-century technology and believed the sheer size of the nation would be more of a constraint than it has been. He also had high hopes for the character of the elected representatives who would preside over this clash of interests.

But the basic message of the Federalist No. 10 is clear, and still relevant. Interest groups are not a threat to our form of government. They are part of our form of government. Their mission is not to help the federal government run smoothly, but to influence its course and keep it from being overbearing, even in its changes of direction. The interest groups help protect us from ourselves.

There is no question that the political and economic fight that is about to begin over the Reagan administration's programs will be long, tough and possibly nasty. That will undoubtedly upset some people. But we shouldn't rule out the possibility that the struggle will invigorate at least some of our interest groups and some of our political leadership, and clarify rather than confuse our various understandings of the national interest. Madison's ghost may even be smiling.