Is pivotal W. German Liberal Party heading toward extinction?

Are the Liberals on their way to extinction? And if so, will West German politics lose its stability? These questions hung in the air at the dispirited convention of the small but crucial Liberal Party June 1 to 3 in Munster. Veteran party chairman (and Foreign Minister) Hans-Dietrich Genscher caught the mood when he told the convention that the Liberals are going through ''the most difficult period'' in their history.

Even German observers sympathetic to the party - a declining species of their own - are voicing doubts these days. In the past, the crisis-ridden Liberals have often enough risen from their ashes. But this time around they seem to be losing their energies of renewal as well as their sense of identity.

The consequences could be profound for West Germany. For decades it has been the pivotal Free Democratic (Liberal) Party that has determined whether right or left would rule West Germany - and has then moderated the excesses of that coalition right or left to produce a comfortable German middle.

One of the few German journalists still sympathetic toward the party sums up its plight in describing it as ''a head without feet'' and ''brakes without a motor.''

He is referring, first, to the party's precarious continued survival at the federal level despite falling under the minimum 5 percent for parliamentary representation in 6 out of the country's 11 states in recent elections. He is also referring to the reduction of the Liberals' role from their once-upon-a-time origination of positive new concepts to little more than a negative check on their coalition partners.

More broadly, he is pinpointing the loss of identity of a party that, after two shifts of coalition, perceives itself as having been too clever by half and having ended up with all tactics and no principles. A sizable chunk of the party's free-enterprise right wing abandoned the party with its first switch from a center-right coalition to a center-left coalition in 1969 - and never came back.

Another chunk of the party's civil-rights left wing abandoned the party with its return to a center-right coalition in 1982 - with some of the brightest young Liberal leaders defecting to the Social Democrats.

As a result the Liberals will be fighting for survival in two weeks in the European Parliament test vote and next year in elections in North Rhine-Westphalia and in the only two states in which they are still members of a ruling (conservative) coalition: Berlin and the Saarland.

The Liberals' initial points of identity within the 19-month-old federal coalition involved (1) stimulating business and (2) assuring the continuity of the popular East-West German detente begun by the left-center government and long resisted by the conservatives.

The current recovery from the several-year recession will either rebound to the conservatives, however, or founder under the current strikes. In either case it will not help the Liberals. And by now the conservatives have themselves embraced ''Ostpolitik'' so thoroughly that there is no glory left here for the Liberals either.

The mood in the country is now such that many in both right and left would welcome the demise of the Liberals. On the right, Bavarian Premier Franz Josef Strauss thinks his Christian Social Union and Chancellor Helmut Kohl's Christian Democrats together could win a majority on their own and not have to accommodate to the ''brakes'' of the Liberals.

On the left, many Social Democrats still want revenge for what they see as the Liberals' betrayal of the Social Democrats in 1982.

The disappearance of the Liberals could shake West Germany's long political stability. If the Conservatives did win a majority on their own, there would be no strong check on the conservatives' own right wing and on the redoubtable Strauss, a man who rouses more antagonism among north Germans than any other politician.

If, on the other hand, neither the conservatives nor the Social Democrats won a majority, and the Greens held the balance, the Social Democrats would swing left - and antagonize both north and south Germans.

Either way, the result would be the kind of political polarization that greatly disturbs West Germans.