If predictions freely circulating six months ago had held true, by this time the government of West German Chancellor Helmut Schmidt should be lying in ruins. The left-wing peaceniks of the Social Democratic Party should have repudiated Schmidt's support for the famous NATO "double- track decision" to proceed with deployment of the U.S. Cruise and Pershing missiles while negotiating with the Soviets for the elimination of such weapons on both sides. And the chancellor, presumably, would have made good on his threat to resign.

But last week's SDP party conference did not live up to advance billings. Resolutions most damaging to NATO and American defense designs for Europe were comfortably defeated. And so the postmortems here on the Munich meeting have mostly to do with Schmidt's skillful give and take, his firm hand on party controls. There's just one hitch: the party Schmidt so masterfully controls is shrinking fast in the polls. It had a mere 31.7 percent approval rating just a week or so ago, while its opposition in the governing Bundestag, the Christian Democrats, climbed up over the 50 percent mark.

Munich, in short, has done nothing to quiet speculation here on the crucial question: can Helmut Schmidt hang on, hoping for some economic miracle that might reverse his party's fortunes, until the next federal elections in October 1984? By the look of it, nothing but trouble confronts Schmidt. State elections are set for Hamburg in June and Hesse in September. In the most recent state vote, in Lower Saxony, the Christian Democrats won with more than 50 percent; the Social Democrats hit a 27-year low, at 36 percent. If that trend holds, the Christian Democrats would win Hamburg, narrowly, and Hesse, handsomely. These elections determine the makeup of the Bundesrat, an upper house with little influence. But the loss by the SDP of Hesse, even without Hamburg, could give the Christian Democrats a blocking two-thirds legislative veto--the theoretical power to paralyze government.

Even so, it would not be easy to bring about new elections. A more likely prospect, if paralysis set in, would be a collapse of the current coalition, with the leader of the Free Democrats, Foreign Minister Hans- Dietrich Genscher, breaking away to give the Christian Democrats a majority. But here again the plot thickens. The defused nuclear issue has given way to economics as the likeliest breakaway issue, specifically the Socialists' policy of higher taxes for "job creation" to deal with the 8 percent unemployment. There are good reasons Genscher still might not want to take the plunge. His own Free Democrats would be split; he would risk alienating left-wing supporters, in the party and the electorate. And the Christian Democrats are not uniformly keen to take power just yet.

Schmidt may have come across last week as every inch the captain--of a foundering SDP. But the further message from Munich is that while he may hang on to the helm until 1984, he is likely to be less and less the master of his coalition government's fate.