The conventional worry about superpower summits -- among those opposed to them -- is that they create an atmosphere of unrealistic expectations about U.S.-Soviet relations and lull unsuspecting Americans into believing that the Soviets have changed their ways.

On the eve of the Washington meeting between President Reagan and Soviet leader Mikhail Gorbachev, the antisummit apostles again have taken counsel of their fears. They worry that an era of good feeling will lead to a dangerous decline in the U.S. military budget and eventual cancellation of the Strategic Defense Initiative (SDI), Reagan's missile-defense plan.

Ironically, apprehension about this summit is particularly keen among conservatives who have followed Reagan's banner up hill and down dale and often exaggerated his abilities. Now they behave as if Gorbachev were 10 feet tall and view Reagan as a diminished leader who seeks to restore a tarnished presidency by posing as a peacemaker.

After claiming for two decades that Reagan was a shrewd and resolute leader with impeccable anticommunist credentials, the president's core constituency seems to have accepted the liberal line that Reagan is made of Teflon and tinsel. They are worried that he cannot hold his own against Gorbachev and fear that the Soviet leader will sway Americans weary of the economic burden of the U.S. military buildup.

Paul Weyrich of the Free Congress Foundation, a thoughtful leader of the antisummit school, put it this way recently: "Reagan is a weakened president, weakened in spirit as well as in clout and not in a position to make judgments about Gorbachev at this time." Weyrich believes that the best hope of saving the SDI is that "the Soviet Union will be the Soviet Union" and take an action akin to the 1979 invasion of Afghanistan that reminds the world of the incorrigible nature of the communist system.

Perhaps it is Reagan's fate to be always popular and always underestimated. It is easy to like him. His charm, optimism and persistence have a contagious quality. It is also easy to underrate him, for his intellect is unimpressive and his inattention to detail legendary. He almost never acknowledges a mistake, and he does not prepare as he should. During the presummit period, he supposedly read books about South Africa and China instead of Gorbachev's revealing new book, "Perestroika."

Reagan has been able to transcend his limitations because he has a usually reliable political instinct about when it is time to strike out in new directions and attempt the impossible. He also has a better sense of what he wants to accomplish than do ordinary politicians. For a great many years, Reagan has believed that the inherent inefficiencies of the backward Soviet system would bring the Soviets to the nuclear bargaining table if the United States showed determination to build up its military power.

The fruits of that idea have ripened into the intermediate-range nuclear forces (INF) agreement that Reagan and Gorbachev are to sign Tuesday. The accord, of value in its own right, is most important for establishing a precedent for on-site inspection that would have been unthinkable a few years ago.

Sure, there are dangers in being dazzled by Gorbachev's smile or in thinking that the Soviets are "just like us" in honoring international agreements. Yes, it is important for Reagan to take note, as he has and will, of murderous Soviet conduct in Afghanistan. Certainly, future American presidents will need creative leadership to maintain military readiness in a world less suspicious of the Soviets.

But, in dealing realistically with the new Soviet leadership, Reagan is recognizing that there are greater dangers than honest bargaining. He is showing faith in the American people, who are unlikely to rush headlong into unilateral disarmament because leaders of the rival superpowers have taken a small step to reduce their nuclear arsenals.

The dangers of a summit pale when compared to the dangers of an unending arms race that could end in the unimaginable horror of nuclear war. It is time for Reagan's followers to join him in taking counsel of their hopes, rather than of their fears.

Reaganism of the Week: Answering questions from high school students in Jacksonville last Tuesday, the president said he didn't mind that Gorbachev was popular and added: "No, I don't resent his popularity or anything else. Good Lord, I costarred with Errol Flynn once."