THE THIRD time seems to be the charm for Ronald Reagan and Mikhail Gorbachev. Their first encounter in Geneva two years ago was little more than a chance for the leaders of the two superpowers to get to know eachother and produced no tangible results. Their last meeting, last fall in Reykjavik, Iceland, was a disaster. Before his aides interceded, an unprepared and uncomprehending Reagan agreed in principle to the dismantling of the entire American arsenal if only the Soviets acquiesced to his desire to proceed with the Strategic Defense Initiative. Now, one year later, the two will sign a treaty that will lead to the abolition of intermediate-range missile from the European continent and lay the groundwork for a more orderly diminution of the superpowers's strategic arsenals.

Gorbachev arrives in Washington today riding a remarkable wave of popularity among Americans. Twice as many Americans view him favorably than view him unfavorably. That's a better percentage than any of the current crop of presidential candidates. Americans should cheer his visit and be heartened by his apparent willingness to deal with the U.S. in a straightforward, productive manner. At the same time, though, their impressions should be tempered by the recognition that he heads a regime, if not a nation, whose values are in fundamental opposition to our own.

Doubters need only look at Soviet human rights policies at home and abroad. But such policies do not--and cannot--preclude the two superpowers from hammering out a decent, fruitful and long-lasting working relationsip. Reagan's conversion to this point of view is one of the more remarkable twists of recent history.

One of the more entertaining sideshows of the summit has been put on by those unwilling to concede even this possibility. Five out of the six candidates for the Republican presidential nomination oppose the treaty, under which the Soviets will dismantle three times as many warheads as the West. That's not good enough. The most conservative president of this century, it turns out, is no more than a "useful idiot for Kremlin propaganda," according to the head of the Conservative Caucus, one of the president's earliest and (until now) most ardent supporters.

Critics on the extreme right fear the loss of the weaponry they say has deterred a Soviet invasion of Western Europe since the end of World War II. Yet the missiles being removed are aimed at the Soviet Union. The tactical weapons that would be used against a Soviet conventional attack remain. Furthermore, such arguments assume that only nuclear weapons--and not commonsense--account for 40 years of peace. Reagan himself put it best in what must have been a wounding retort to his critics on the right: "Whether they realize it or not, those people--basically, down in their deepest thoughts--have accepted that war is inevitable and that there must come to be a war between the two superpowers.

One need not be blinded by Gorby-fever to reach the opposite conclusion. Reagan's motives in pushing for the treaty have been impugned by his critics, who interpret his actions as those of a man egged on by a wife solely concerned with her husband's place in the history books--as if that would in any way detract from what on the merits clearly is a sensible, long-overdue reduction in nuclear weaponry or if the judgment of history is a factor to be dismissed in the conduct of affairs of state. If anything, the virulence and mean-spiritedness of the opposition emanating from those on the President's right reveals that they themselves may have realized that history has passed them by.