America's best friend in the world today is probably the visiting chancellor of West Germany, Helmut Schmidt. But Herr Schmidt currently faces grave political troubles at home.

So the Reagan administration ought to break its nearly total concentration on domestic affairs and think hard about what it can do to help him. Like, maybe, having President Reagan engaged at this time to visit Germany as Schmidt's guest.

West Germany's role in the world defines, in part, Schmidt's special importance. The Federal Republic boasts the strongest economy in Europe and the most impressive conventional forces. It fronts on the Soviet bloc, and it is geographically central to the defense of Western Europe. It maintains multiple ties with Eastern Europe, and leads the way in working to loosen Soviet control over other nations in the Communist bloc, especially Poland.

The weak condition of other leading European allies serves to enhance Schmidt's value to the United States. Margaret Thatcher, with everything she can do to manage Britain's domestic economy, is of limited use as an ally. Francois Mitterrand, the newly elected president of France, has yet to define his regime and its relation with the Communist Party. If he names Communist ministers, Italy may be tempted to follow suit. Even if he doesn't, he will probably come out in opposition to the United States in such areas as Latin America and Africa.

The internal troubles burdening Schmidt arise in large measure from his willingness to stand up as a friend of the United States in its struggle against the Soviet Union. He went further than any other European leader in applying sanctions against Russia after the invasion of Afghanistan. In December 1979, he played the key role in a NATO decision to offset the Soviet buildup in Eastern Europe by modernization of Theater Nuclear Forces (TNF) in Western Europe.

The TNF decision provides that 572 medium-range nuclear missiles will be deployed by 1983. West Germany joined with Britain and Italy in offering to let the weapons be stationed on home ground. Schmidt's only condition was that there be initiated simultaneous arms control negotiations between the United States and Russia to limit deployment on both sides of the intermediate-range missiles.

The opposition to Schmidt in Germany comes chiefly from his own Social Democratic Party, and focuses on the TNF decision. The left wing of the Socialists, grouped around former chancellor Willy Brandt, charges that the decision works to compromise German relations with the East -- including a fair prospect for improving ties with East Germany. More recently, as part of the attack, the left-wingers have been asserting that the Reagan administation has no serious intention of negotiating an arms control accord with Russia. It is notable that one Socialist member of the Parliament, Manfred Coppik, said last week that "in the present situation the main danger to the peace comes from the policy of the U.S."

The spread of such views could easily deal a quick death blow to the chancellor. The Socialists govern in coalition with a much smaller Liberal party. The Liberals are strongly committed to the TNF decision. If they become convinced the Socialists were going to renege, they would switch sides and join the Christian Democratic opposition in establishing a new government. The upshot not only would be the ouster of Schmidt, but also would mean the conversion of the Socialists to die-hard opposition to nuclear modernization. It might even put them at odds with the Atlantic connection.

Just before coming to Washington Schmidt force the issue to a head. In a series of speeches he made it plain he was ready to resign if the Socialists went back on nuclear modernization. "I stand and I fall," he said to one party meeting of the TNF decision. To another meeting he cried: "Stop once and for all letting anyone suggest to you that the Americans are our enemies and the Soviets our friends."

Even the threat to resign may not be enough to stem the ride. Some Socialists probably want to go into opposition. Other Germans, including the Liberals, will stay with Schmidt only if they are convinced the United States stands behind them. So the chancellor's survival is bound up with what he achieves here in Washington.

The Reagan administration has already taken some steps on his behalf. As an earnest of good faith on arms control negotiation, Secretary of State Alexander Haig last week began talking about the terms of negotiation with Soviet Ambassador Anatoliy Dobrynin. A truly distinguished American -- former Federal Reserve Board chairman Arthur Burns -- has been appointed ambassador to Bonn.

But with so much at stake, with Europe slipping toward neutralism, a far larger gesture is in order. Ideally what the president should do to salvage Schmidt is to commit himself now to participate in the arms control discussions next year. If that proves impractical, a good symbol would be a pledge at this time to visit Germany some time in 1982, perhaps on the 35th anniversary of that high point in Atlantic relations -- the Marshall Plan.