This is a promising period in the twilight of the Reagan presidency. It is also a critical time in U.S.-Soviet relations, as the administration struggles to regain its prestige and make the most of arms-control opportunities likely to expire with Reagan's second term.

Reagan's secret strength, which some call his "luck," is an ability to recover from adversity. He is at his best with his back to the wall. He rebounded from political reverses in 1976 and 1980, from a near-mortal bullet wound in 1981, from the busted summit in Reykjavik last year and, most recently, from the high-risk gamble of trading U.S. arms for American hostages.

Reagan is less adept when success seems in his grasp. He allowed his 1981 income-tax reduction to become a grab-bag for special interests. He forfeited the high ground of his first term with an empty reelection campaign, producing a landslide without a mandate. He failed to resolve policy conflicts within his administration, costing him the chance to benefit from the opening he helped to create at the 1985 Geneva summit.

Now, Reagan has another chance. The intermediate-range nuclear forces (INF) treaty being tied down by U.S. and Soviet negotiators will scrap an entire class of nuclear missiles. It opens the prospect of a more far-reaching accord that would reduce the superpower strategic nuclear arsenals. A fall summit with Soviet leader Mikhail Gorbachev will also enable Reagan to keep the spotlight on U.S. demands that the Soviets withdraw from Afghanistan and free rank-and-file dissidents.

But the president is under pressure from conservatives who view any arms agreement as a sign of surrender. This has dimmed his public enthusiasm for the achievement of his negotiators at a time when U.S. signals are important to the Soviets.

On the day that Reagan announced the INF treaty, the Defense Department tried to upstage him by proclaiming an acceleration of efforts on key technologies of the Strategic Defense Initiative (SDI), the "Star Wars" missile-defense plan. The SDI declaration was cheered by those who want to put a defense system prematurely into space on the assumption that it will shoot down a prospective treaty even if it can't stop Soviet missiles.

Three days later, Reagan gave a U.N. speech that tried to turn Gorbachev's policy of glasnost, or openness, against the Soviet leader. Going beyond the requisite defense of human rights, Reagan seemed to be urging the Soviets to become a U.S. replica by allowing "political and intellectual liberty in all its dimensions."

Tass understood the message. "The U.S. president, while claiming to be interested in changes happening in the Soviet Union, gave his own interpretation of glasnost, limiting it to a list of demands on the Soviet Union," the Soviet news agency complained. "There was not even a hint of any kind of readiness for a change in thinking and politics from the American side."

It may be time for a change, even if Tass favors it. Reagan's firmness is not in doubt, either on human rights or Afghanistan. The SDI has become so widely accepted that its opponents have been reduced to arguing about how much they should spend on it.

Traditionally, Soviet negotiators stand firm, waiting to see if they can wrest concessions from American leaders who, in former Pentagon official Richard N. Perle's useful phrase, value arms control as "incantation." This time, the U.S. side held its ground, confident that the Soviets wanted a treaty and a summit and would accept the U.S. proposal.

The premise was correct, and it may be time for Reagan to risk the wrath of the right-wingers and claim victory. It may also be time for him to recognize political reality and pledge adherence to a strict interpretation of the 1972 Antiballistic Missile (ABM) treaty before he is forced to do so by the Senate.

Reagan doesn't have to rush the SDI into space to save it or push glasnost to its outer limits to demonstrate his anti-Soviet credentials. Those are not in doubt. What is in doubt is his ability to forge an administration consensus and convert his opportunity into meaningful agreements. Reagan has once more eluded political oblivion. He needs now to show that he has learned the lessons of the past.

Reaganism of the Week: Speaking to the U.N. General Assembly last Monday, the president said, "I occasionally think how our differences worldwide would vanish if we were facing an alien threat from outside this world."