Bonn coalition totters, foreign policies endure

''Nothing will change but the rhetoric.''

This is the almost universal reply of diplomats and politicians asked about how a conservative government in Bonn might alter foreign policy after 13 years of Social Democrat-Liberal rule.

US-European differences would still rankle, the consensus runs. As chancellor , Christian Democratic Union leader Helmut Kohl would be as eager as current Chancellor Helmut Schmidt to hold summit meetings with Soviet leaders. The new NATO missiles would still be deployed in the mid-1980s - but with less West German pressure on the United States to agree on arms control with the Soviet Union and with greater domestic West German opposition to the missiles.

In the one foreign policy area that the conservatives might be inclined to change - relations with East Germany - they would be essentially locked into the status quo.

Despite the general agreement that there would be little new in conservative foreign policy, speculation about it generates endless discussion in Bonn these days as verbal warfare escalates between the two partners in the governing coalition. The expectation is that within a few weeks the junior liberals, who hold the swing vote in the Bundestag (National Assembly), will switch their alliance from Social Democrat Schmidt to Christian Democrat Kohl.

Under Helmut Kohl the rhetoric would change. That much is clear. The West German conservatives accuse the Social Democrats of damaging relations with the US, saying they would do better. In an earlier period of the Reagan administration they had even hoped that such conservative solidarity might encourage Washington to help topple Schmidt and install a Christian Democrat chancellor here instead.

Today no such American help is needed. And it is not clear that the White House would find the new man in the chancellor's bungalow any more forthcoming than Schmidt on today's key US-European quarrels: the Siberian gas pipeline and trade protectionism.

On the gas pipeline, West German conservatives consider Reagan's application of a unilateral American embargo to European subsidiaries and licensees of American companies as illegal as do the Social Democrats. The conservatives have been equally strong in defending the sanctity of the European contracts for Soviet pipeline construction signed before the American embargo. They - like West German businessmen in general - are equally dubious about the whole idea of Western economic sanctions on the Soviet Union and Poland.

As proof of this thesis one need look no further than Economics Minister Otto Lambsdorff, an Americanophile, the stalking horse of the projected liberal switch to the conservatives and a public advocate of far more conservative domestic policies (in cutting social welfare) than even the Christian Democrats endorse. This staunch friend of America and of American conservatives was the first West German official to protest Reagan's extension of unilateral American sanctions to cover European firms as well.

On the pipeline issue even the rhetoric would not change very much under the conservatives, diplomats say. Chancellor Schmidt has remained in the background on this issue and let French President Mitterrand and British Prime Minister Margaret Thatcher lead the confrontation. Helmut Kohl would no doubt do the same.

Similarly, in the transatlantic imports feud that all the professionals devoutly hope will not escalate into a trade war, the West German conservatives' heart is with Europe. Here the Reagan administration has been trying to soften the steel clash - and the clash over barriers to American food exports to Europe is not an acute issue at the moment.

On East-West relations, observers here expect that a conservative West German government would be more modest than the Social Democrats have been in hopes for detente. Nonetheless, West Germany's front-line geographical position and its dependence on some cooperation to ensure East-West German personal contacts predispose all West German politicians to regard ideological confrontation - as sometimes preached by the Reagan administration - as a dead end.

A signal that detente would continue to be the West German goal has, in fact, just been given - or so it is interpreted here - by liberal Foreign Minister Hans-Dietrich Genscher. Mr. Genscher - who is expected to continue as foreign minister under a shift of coalition - has an article in the forthcoming issue of Foreign Affairs endorsing detente.

The one group of diplomats that has doubts about a conservative policy on East-West relations is - not surprisingly - the East Germans. Western diplomats report being asked by East German colleagues if Kohl would be like the moderate wing of his party that accepted detente in the 1970s - or if he would be like Reagan.