Helmut Schmidt won, so why is he unhappy?

West German Chancellor Helmut Schmidt, with a fresh mandate of a 45-seat majority in parliament, is down in the dumps. Opposition leader Helmut Kohl, whose party just made the worst showing in postwar history, is riding high.

This is one of those paradoxes that confound outside observers. And its irony is heightened by the conservative opposition's choice of this twilight period in East-West detente to approve, finally, the decade-old West German government policy of detente.

Blame it all on the end of West Germany's economic miracle, perhaps, and the drying up of extra money to pay for new social reforms or even all the old social welfare. Or blame it on unrealistic popular expectations of what the government can do -- and on crisis-manager Schmidt's plain speaking about the limits to what he can do.

Or blame it on the fact that Mr. Schmidt's October electoral gains were really won by the junior coalition party, the Liberals, and schmidt has therefore had to veer more toward the Liberals in labor-relations issues and away from his longtime trade-unions allies.

Whatever the cause, the ruling Social Democrats' malaise surfaced with Schmidt's keynote address to the Bundestag on Nov. 24. It was a laundry list of government policies -- and nothing more. The German press panned it and speculated about the chancellor's "loss of authority."

Dr. Kohl (to good press reviews) charged that it failed to give the country "intellectual leadership." Even Schmidt's Social Democratic Party (SPD) colleague and ex-Chancellor willy Brandt warned that "those who bear political responsibility must remain open to new ideas."

The Young Social Democrats -- who during and since the election have been forced by their parent party to restrain their activist leftist enthusiasms -- stewed and said as little as possible about the speech.

In the chancellery a "melancholy" Schmidt was overheard wondering if he would last until Easter -- or so the news magazines reported. (A few skeptical observers noted that Schmidt has been saying he was going to resign ever since he was finance minister, however, and Schmidt himself finally shot down the report after letting it float for a week or so.)

In any case, at least some of the Social Democratic gloom seems to partake of the old Chinese supertistion of not saying aloud how well things are going for fear of drawing evil spirits' attention and attacks. Fickle moods are probably a less-reliable barometer of current politics than is the conservative Christian Democrats' (CDU) embrace of SPD-Liberal detente.

This turnabout was conspicuous in the parliamentary reply of CDU chairman Kohl to Schmidt's government declaration. In his speech Mr. Kohl went well beyond the Conservatives' previous grudging pledge simply to honor binding international East-West treaties if it came to power. More warmly, Mr. Kohl said that the various detente treaties are important components of West German foreign policy. Kohl also effectively buried the Conservatives' campaign alarms about a "Moscow faction" in the SPD.

What changed Kohl's mind about detente -- in an unpropitious year of a Soviet occupation of Afghanistan and threat to occupy Poland -- was the resounding defeat of the West German Conservatives in the October election. The voters rejected chancellor candidate Franz Josef Strauss, head of the CDU's more conservative Bavarian partner, the Christian Social Union -- and with him they rejected a right-wing alternative to the present middle-of-the-road consensus.

The axiom of German politics of the past quarter century was once again proved: Neither of the large parties can attain a majority without allying with the small, pivotal Liberals. The only way the Conseratives can win back the government and power that has eluded them for the past 11 years is to woo the Liberals away from the Social Democrats --to become more middle-of-the-road themselves. Hence the belated shift to favor detente.

Not all Conservatives agree with this shift. Mr. Strauss's CSU still holds to its old suspicions of detente. So do some CDU members like defense spokesman Manfred Woerner.

The conciliatory main line has been set forth by Kohl, however -- and it will mean an easier time for the Social Democrats in foreign policy, despite their gloomy mood.