At a time when all good cold warriors have rearmed and remounted, a curious thing has happened to West Germany's leading anticommunist political knight. He will not put on his armour.

Franz Josef Strauss, the Bavarian state premier who is a candidate in this autumn's elections against Chancellor Helmutt Schmidt, has surprised pundits and public alike here by refraining from his customary anti-Soviet fulminating.

While uttering a few "I-told-you-so's" about the invasion of Afghanistan, Strauss has shown some reluctance in backing America's tough sanctions and placed a solid accent, like Schmidt, on pursuing detente.

In fact, many West Germans say they are having difficulty distinguishing between Strauss and Schmidt on foreign policy. This has made for, among other things, much less campaign fun than everyone had expected.

The 64-year-old Strauss has been a major figure on the right wing of West German politics for virtually all of the postwar era. He is the longtime head of the Christian Socialist Union in Bavaria, the sister party to the vastly larger and generally more centristt Christian Democratic Union, which operates in nine other states. He is regarded as intelligent and politically gifted, with a good sense of humor.

His nomination as the opposition's candidate for chancellor promised to provide West Germany with the clearest choice ever between conservative policies and those of the left-of-center coalition that has ruled here since 1969.

That was before Afghanistan, when issues such as nuclear power, the economy and taxes were expected to dominate the campaign. But the Soviet invasion has taken over the headlines here as in America and on the question of how Bonn should respond, Strauss appeared, especially at first, to be seeking a balance between defense and detente.

This is very close to Schmidt's position. Strauss even managed to bring Schmidt together with other opposition leaders who earlier had taken a harder stance in a private session to discuss the crisis.

Lately, as independent polls show Schmidt's Social Democratic Party gaining and the opposition losing slightly in popularity, Strauss adopted a sharper tack.

He endorsed a boycott of the Moscow Olympics together with broader economic sanctions against the Soviets and a larger increase in West Germany's defense spending than the rise of 3 percent in real terms that the government begrudgingly has agreed to. Strauss would double the number of frigates in the West German Navy and have Europe develop its own long-distance military air transport capability.

Openly upset by a recent American press report suggesting he had turned into an appeaser of the Communists, Strauss sought to clarify his position during a lunch with U.S. correspondents this week. The lunch was in anticipation of Strauss' visit to the United States, which begins Saturday and will include a meeting with President Carter next week.

"The role of cold warrior is so identified with me that I don't have to apologize," he said. "My reputation in Moscow is not the most favorable.

Still, while Schmidt and other senior Bonn officials were forced by circumstances last month to postpone visits to and from East European officials, Strauss dropped in on Romanian party chief Nicolae Ceausescu in Bucharest.

The trip prompted political observers to ask whether Strauss had undergone a miraculous transformation or simply was practicing shrewd election tactics? Where, it was queried, was that choleric rostrum-pounder Strauss, who in 1974 said. "What we need here in this country is the courageous citizen who chases the red rats where they belong."

The answer appears to be that he is reading the same public opinion polls as Schmidt, which show West Germans to be more worried about the prospects for peace than at any time since the 1961 Berlin crisis. Strauss apparently is engaged in a contest with Schmidt over who can look more statesmanlike.

"As a candidate, I must speak a different language than as a leader in parliament," he explained at lunch. "I must be careful and moderate in my statements."

With that, he warmed up, attacking Schmidt's government for having "confused and poisoned West Germany with a romantic illusion of detente," and for giving the word "solidarity" a "bad aftertaste." He said he prefers the term "common interest."

But, like Schmidt, he criticized Carter for failing in the past to consult enough with the NATO allies in matters that affected Europe. He also left the door open to the Soviet Union by reiteratng a willingness to visit Moscow if the Kremlin would invite him -- although he asserted that he has "no intention of changing" his attitude.

What is the difference, he was asked, between what he calls his "realistic" detente and Schmidt's "romanticism"? The difference is I see detente not as an end in itself but as a method to reduce the conflict," he said.