Essen, W. Germany — Aided by Soviet stonewalling on arms control, the Social Democrats have more or less slipped back into West Germany's security consensus. But enough ambiguity remains for some veteran Social Democrats to warn the party against drifting away from the Western alliance.
Certainly the Social Democratic Party (SPD) pledged its support for the North Atlantic Treaty Organization yet again at its convention in Essen May 17 to 21. The SPD played down the issue of NATO Euromis-siles - the centerpiece of a special convention last November with its vote of some 400 to 14 to reject the 1984-1988 NATO deployments.
And the SPD, while calling for an undefined new strategy for NATO, postponed until 1986 any specific proposals for an ''alternative'' strategy that a number of its members would like to see move away from large Army divisions toward small-unit sharpshooters.
All this bitterly disappoints peace activists - and many young leftist Social Democrats. The most comfort they could get was from the SPD's denunciation of nuclear deterrence, its warning (to the West) not to mount a conventional arms race, and a concrete resolve to set 1983 defense outlays (as a percentage of the federal budget) as a ceiling on future military spending.
The best measure of the shift away from last November's antinuclear mood was the rebuttal of critics of this year's middle-of-the-road security resolution by Erhard Eppler. Mr. Eppler, a member of the convention's resolutions committee and a longtime opponent of Euromissiles out of Lutheran conviction, was one of the chief actors pushing the party away from its old, reluctant tolerance of the new NATO missiles last fall.
This time Eppler chided Lutheran pastor and peace activist Volkmar Deile for mistrusting the Social Democrats. In an open letter Deile had reproached the SPD for not setting forth how it plans to reach its goal of ''overcoming the blocs'' (NATO and the Warsaw Pact) and for not vowing to ''carry through'' West German interests in NATO rather than just representing them.
Does this mean the pragmatic defense wing has won back the party? Only provisionally.
Those with more radical views on peace and defense deem this as a period of temporary retreat: A Moscow that is boycotting all nuclear arms control talks hardly offers them a lever with which to pry policy leftward just now, and the German voter is currently far more preoccupied with labor strikes than with missile strikes.
The peace activists in the SPD also view their chances as greater in the future now that ex-Chancellor Helmut Schmidt, that bulwark of SPD pragmatism, has retired from party politics. At the Essen convention Schmidt dropped his last SPD post as party vice-chairman.
The situation is tenuous enough to have elicited a warning from Berlin Free University's Richard Lowenthal in the run-up to the convention. Writing in the weekly Die Zeit, he took SPD administrative chief Peter Glotz to task for in effect inflating anti-Reagan reactions (which Lowenthal approves) to estrangement from America (which Lowenthal emphatically repudiates).
Mr. Glotz incorrectly sees fundamental clashes between US and West German interests, Lowenthal said, where there are only resolvable differences of policy. Glotz's complaint that the ''Western links of the (West German) Federal Republic'' mean a negative ''embedding of German policy in the interests of the Americans,'' Lowenthal argued, leads to fallacious positing of German identity as hanging between East and West.
Some Social Democrats' dismissal of danger from Moscow, Lowenthal adds, obscures the fact that Europe's post-war stability rests on maintaining an East-West military balance rather than on Soviet good will.