Out of the Swabian hills of southern Germany, where philosophical fancy and stubbornness are considered something of a local hallmark, comes Erhard Eppler, main voice of West Germany's peace movement and one of the prime adversaries of Chancellor Helmut Schmidt.

During the 1960s, when he and fellow Social Democrat Schmidt were members of the Bonn parliament, "there was no tension at all between us. We had a good working relationship," says Eppler. Today, the 54-year-old former teacher rides the crest of opposition here to Atlantic Alliance plans to deploy medium-range nuclear missiles in Western Europe -- plans on which Schmidt has staked his office.

Eppler's skill as an orator and his history as a high-ranking party maverick -- he belongs to the Social Democrats' 13-member executive committee -- have put him much in demand for peace rallies and televised debates. He is a key figure in a movement that has few notable leading personalities.

Some have suggested that Eppler and Schmidt may have more in common than either would care to admit. Both are smart, Protestant sons of schoolteachers and both can be arrogant and impatient.

But Eppler dismisses such comparisons. "His state of consciousness is completely different from mine," Eppler said of the chancellor. "He thinks in terms of restoring a past society that cannot be restored. I think in terms of the society of the 1980s."

He opposes the new missiles, he said in an interview, because he thinks they are unnecessary. He regards them as a dangerous turn in the arms race, and he doesn't trust the Reagan administration to negotiate seriously about arms reductions with the Soviet Union.

Instead of the existence of two heavily armed blocs in Europe, Eppler advocates a nuclear-free zone in Eastern and Western Europe, as suggested in 1957 by Poland's then-foreign minister Adam Rapacki. Western officials consider the idea unrealistic.

Eppler drew a parallel between the peace movement in Western Europe and the upheaval in Poland. He said the trouble the Soviets were having with the Poles was similar to America's troubles with the European protesters, and described both developments as part of "the Europeanization of Europe."

Asked whether a new, specifically German patriotism was also being expressed through the protests, Eppler said that is certainly an element and should be considered natural. He went on to warn that the "only chance of creating a strong movement for German neutrality is to tell us we have no interests of our own. The only man who can create neutralism in Germany is not Soviet President Leonid Brezhnev but President Reagan, and he's been very successful at that so far."

He apparently was referring to a recent remark by Reagan, who suggested that a limited nuclear war in Europe was possible. The comment provoked strong media reaction in Europe.

A deep idealism, often laid to his Swabian roots, runs through much of what Eppler has had to say, not just on security matters but on ecological, energy and Third World issues as well.

"We all pride ourselves far too much on being realists," he wrote in 1975 in his book, "End or Turning Point." "We just smile at dreamers. True, there is no responsible way around reality. True, wishful thinking is sure to avenge itself. Yet realism alone is stale when taken on his own and not exposed to the tension of utopia."

An earlier book, entitled "Too Little Time for the Third World," was written in 1971 when he was serving as Bonn's development aid minister. He dates his radicalization from that time, saying that contact with the Third World "changed my attitude toward things in the first world."

It was a clash with Schmidt over the level of West German foreign aid that led Eppler to leave the Bonn government seven weeks after Schmidt became chancellor in 1974. But he is determined in the current clash, he said, not to quit the party.

He sees his role as keeping the Social Democratic Party from losing ground to alternative left-wing political groups that have emerged in West Germany and are involved in the peace movement.

The largest of these groups, called "The Greens," made gains, among other places, in last year's Baden-Wuerttemberg state elections, prompting Eppler to resign his state party chairmanship there. Eppler's interest now in reaching out to such groups appears to ally him with former chancellor Willy Brandt, the party chairman, with whom Eppler feels both a personal and political kinship.

Eppler's bridging effort has made him suspicious to both sides. Some party officials accuse him of using the peace movement to further his career in the party or build a new party base, while some peace movement groups suspect he may be seeking to co-opt them into the party. At some point, his critics say, Eppler will have to choose sides.

A key test for him and Schmidt will come at the Social Democrats' national congress next April. Two years ago, at another party congress, Eppler led a drive to scale down the development of peaceful nuclear power in West Germany. He won 41 percent of the delegate vote then against Schmidt's more pronuclear policy. The question now is whether even more than those who supported him in the earlier challenge will back his antimissile stance.