In one sense the Reagan White House was probably better off not granting an audience to Petra Kelly, the most vocal and visible driving force of West Germany's radical and rambunctious Greens. The irony might have been too thick to cut. For if the Green Party's ends are essentially West German in origin and impetus, its means and methods were made in America. To confront Kelly is to relive the Vietnam and civil rights movements of the '60s in the United States when Kelly, as it happens, was beginning her political activism while studying at American University.

She formed a student group for Robert F. Kennedy's presidential campaign and, by her account, "learned all the experience" of Martin Luther King. Returning home, imbued with the power of passive resistance, she first helped organize and then helped lead the Greens from the streets to 5.6 percent of the vote in the last German elections and 28 seats in the Bundestag. So there she and her supporters sit, accepting the system, while preaching wide-scale, nonviolent protest as a means of blocking the deployment of new American missiles, scheduled for the end of this year if no arms- control deal can be made with the Soviets.

Enter the irony: she is a sworn enemy of the Reagan nuclear doctrine. She is also practicing democracy in its most vigorous form. There she is, theoretically, a logical product of the sort of cultural exchange envisaged by Ronald Reagan's celebrated programs for exporting our most cherished democratic values worldwide. And that is why not only the White House but others who did grant the visiting Greens an audience might have been better off if they had given her a better hearing.

Let it quickly be said that there is no arguing with Petra Kelly. To talk to her is to open a floodgate. She has done her homework. She has her warhead numbers and her selected American authorities to cite. She is evenhanded about the Soviet share of the blame. But she is supremely confident about Western superiority--enough so to justify the kind of one-sided disarmament by the West that most conventional experts would think foolhardy.

"It is easy enough to demolish her argument," says a hard-nosed but sympathetic American authority. "But she is impressive in terms of intellectual rigor--once you accept her premise that the world has gone mad." And if you don't? Then the intensity of her rapid-fire delivery and the breathtaking sweep of her assertions invite self-assured snickering in establishmentarian audiences of academicians, government officials, and think-tank specialists. Yet the kind of point-scoring I listened to at one such session seemed to me to miss the basic point: that Petra Kelly represents the main guard of a serious movement with a prevailing dedication to nonviolent protest. That's a plus worth encouraging. The minus is that, like the main guard of any passionate protest movement, the Greens can no more be sure of controlling the vanguard than could the passive resisters of the American 1960s.

This is not a case for appeasing the Greens but, rather, for acknowledging that they constitute a potent and potentially dangerous force, with a wide reach to popular "counterculture" issues (women's rights, ecology, capitalist greed, Third World poverty). They strike me, from just one encounter with Petra Kelly, as strong-minded (however wrongheaded) and dedicated --a force to be treated with some measure of respect.