What is a German liberal, anyway?

The question haunts the preparations for a new government in Bonn as the liberal Free Democratic Party, longtime partner of the Social Democrats of Chancellor Helmut Schmidt, is seen suddenly bolting across the political spectrum to realign itself with the Christian Democrats.

Smallest by far of the three major parties that have controlled postwar West German politics, the Free Democrats, precisely because of their swing function, have tended to hold the balance of power at the federal and regional levels. Their importance has been most often defined in terms of serving as a brake on the leftist tendencies of the Social Democrats or as a progressive force in governments with the Christian Democratic Union.

But this functional definition always begged the question of what the party really stood for. Was it interested more in tactics or programs?

Now, in the face of an angry, determined attempt by Schmidt to discredit the maneuverings of Free Democratic leader Hans-Dietrich Genscher, the party's reason for existing is being challenged as never before, both by voters and by its own dwindling membership.

Founded in 1948, the Free Democratic Party laid claim to a heritage of German liberalism shared by several prewar parties. It won 12 percent of the vote in the first West German elections in 1949 -- a result it has rarely exceeded since.

Today, the party claims about 90,000 members across West Germany, drawing largely from civil service and white-collar workers. According to Rudolf Schwidder, a party spokesman in Bonn, many Free Democrats could be characterized as "social climber, achievement-oriented types." The party's voters, he said, are "predominantly bourgeois."

Over the years, the party's platforms have shown sharp shifts in policy, from conservative to progressive. Asked to sum up Free Democratic policy today, Schwidder said: "The party attempts to occupy the middle of the German party spectrum."

In coalition with the Social Democrats, the Free Democrats sought to steer West Germany toward a less interventionist economic policy at home, and more pro-American, pro-European Community policies abroad. An initiative in which Genscher as foreign minister took much pride was a proposal, still unrealized, to strengthen European political cooperation.

But the party's essential profile and real political weight have generally derived from the positions it could carve out in relation to whatever coalition partner it was with at the time. The Free Democrats have been derided as "the pendulum party" by Social Democratic floor leader Herbert Wehner, as a "depot of opportunism" by Bavarian leader Franz Josef Strauss, and by some, as the "cave-in" party ever since 1961 when, after scoring well by running against then-chancellor Konrad Adenauer, the party agreed to join his Cabinet.

Party officials contend that their susceptibility to switching alliances helps break political crusts and points the way to change in West Germany's staid democracy. But it is often unclear where this presumed interest in change really promotes a turn in some popular new national direction and where it simply masks political opportunism.

In the past, the party was able to lay the groundwork for its jumps gradually and cautiously, to give voters time to adjust to seeing Free Democrats with a new partner. When the liberals last switched federal coalitions, three years elapsed between their divorce in 1966 from the Christian Democrats and the formation of a new coalition with the Social Democrats in 1969.

This time, Genscher is trying to pull his party into a new alliance inside of two weeks. The German press has labeled the switch a "flying change." It is not going down well either with much of the public, which is unsettled by it, or with many Free Democrats, who resent not being consulted.

Reflecting the cost so far, the party lost half the support it had gained four years ago in last Sunday's crucial vote in the state of Hesse. It drew only 3.1 percent, below the 5 percent cutoff to keep seats in the state assembly, and the party's worst showing ever.

Months before the Hesse vote, the party's popularity had begun to wane. Free Democratic officials attributed this to a sense of national fatigue with the quarrelsome Bonn coalition. Fear for the party's existence, always present, as well as genuine differences over policy, caused Genscher to start plotting a change.

Genscher, 55, who is expected to resume his post as foreign minister in the new government, has headed the Free Democratic Party for eight years. He is not a charismatic figure, but works hard, exudes calm and is reputed to have one of the finest-tuned political senses in West Germany.

But this time, he miscalculated. Schmidt caught him off guard Sept. 17 by calling for elections. This upset the phased plan Genscher had of withdrawing from the coalition in October, seeking grass-roots approval for a new center-right coalition at a regular party congress in November, and only then signing a pact with the conservatives.

Now he is being accused by critics in his party of high-handedness and disregard for those who voted Free Democratic in 1980, expecting to have four more years of social-liberal rule in Bonn.

Actually, the Free Democratic Party has generally been run more from above than have the two major parties. This is because the liberals have not managed to increase their number of regular votes. They remain heavily dependent on "situation voters" to leap the 5 percent hurdle for parliamentary representation.

Lack of a strong grass-roots organization contributes to the party's sense of instability and makes its identity largely a function of the profile of its leaders.

One result of this is to make the party appear unusually open and flexible, thus appealing to individualists. But this, in turn, has led to a party membership that probably spans a wider spectrum from left to right than exists among Social Democrats or Christian Democrats.

In the old Bonn coalition, this range was evident in the persons of Economics Minister Otto Lambsdorff, a fierce free marketeer, and Interior Minister Gerhart Baum, a progressive-minded protector of civil rights.

Until now, Genscher had managed to bridge the wings. But the current uproar over his tactics, plus deep-seated left-wing resistance to a new coalition with the right, now threatens to divide the party irreparably.