Greens edge past Liberals in local W. German election

The Greens look as though they will replace the Liberals as West Germany's ''third party.'' That is the clearest result of Sunday's election in North Rhine-Westphalia, a local election that is viewed as the dress rehearsal for next May's vote in this largest state in West Germany and as a harbinger of the next federal election in 1987.

The voters delivered ''a slap on the face,'' as a somber Chancellor Helmut Kohl put it, to his conervative Christian Democrats. And The Social Democrats, despite their best relative showing in 20 years, must still confront the dilemma of whether to aim for voters on the left or center.

The Social Democrats won an overall plurality for the first time in two decades. With 42.5 percent of the vote, they edged out the conservatives at 42.2 percent.

But to move into quite a few city halls - including the one in the state capital, Dusseldorf - the Social Democrats will need the cooperation or the tolerance of the Greens, who shot up from 1.5 percent in the last local election in 1979 to 8.6 percent this year.

The Liberals, the traditional swing party, slipped to 4.8 percent - below the 5 percent minimum for seats.

The results are both an encouragement and a warning to Social Democratic state premier Johannes Rau. He won a precarious majority of seats in the last Landtag election because of the failure of both Liberals and Greens to clear 5 percent - and because of the generally more favorable Social Democratic vote in state over local elections. But he could lose that absolute majority next spring and have to deal with the Greens, as his fellow Social Democratic premier in Hesse has already had to do.

This is an issue of great concern to the Social Democrats on a national level , for they govern only four out of 11 states at present. If they appeal to counterculture Green voters, they risk alienating traditional trade union support - and perpetuating the present conservative majority. If, on the other hand, they keep their unionists by distancing themselves from the Greens, they risk losing their young professionals to the Greens.