Americans believe that the Soviet Union remains a serious threat to the United States and fear the nation is losing ground to the Soviets in world affairs, according to the latest Washington Post-ABC News poll.

Despite those concerns, the public believes that relations between the United States and the Soviet Union are generally good and will get better.

And even though glasnost has produced a thaw in the cold war, American ignorance about U.S.-Soviet relations appears undiluted.

For example: Only a third were able to recall that the two countries were allies in World War II, and slightly more than half said they had read or heard anything about the proposed treaty between the two countries to limit medium- and shorter-range nuclear missiles in Europe, the primary reason for the summit that officially begins here Tuesday.

The survey of 1,007 adults last week found that three out of five Americans consider the Soviet Union to be a serious threat. About three in 10 rated the Soviet Union as a minor threat, while one in 12 said it was "no threat at all." Margin of sampling error for the survey's overall results was plus or minus 3 percent.

Almost two-thirds of those questioned said the Soviet Union has been catching up to the United States in terms of influence in world affairs. But most of those polled believe America remains first in the world: Almost four out of five respondents -- 79 percent -- said the United States has more overall influence on international affairs, despite those Soviet gains.

Americans also consider relations between the two superpowers to be generally good. When asked how they would rate U.S.-Soviet relations, more than half -- 55 percent -- said they were good and 2 percent rated them excellent. Slightly more than a third -- 36 percent -- rated relations as not so good, and 7 percent said they were poor.

Nearly half -- 49 percent -- said they expected U.S. and Soviet relations to get better, while only 12 percent said they would get worse, and 38 percent said they would stay about the same.

In fact, public attitudes toward the Soviets may be less chilly than those expressed recently by Congress. Nearly three out of four respondents -- 73 percent -- said that they approve of allowing Soviet leader Mikhail Gorbachev to speak to Congress.

Those attitudes strongly suggest an American view of the Soviets that is shifting and perhaps maturing. Other results show that the public believes that U.S. policy toward the Soviets should emphasize cooperation on major mutual problems, and not confrontation over longstanding ideological or policy differences.

First among those problems is reduction of nuclear weapons.

Respondents were read brief descriptions "of things that could be a part of this country's policy toward the Soviet Union," then were asked how important each should be to America's policy toward the Soviets.

The list included getting the Soviets to improve human rights in their own country, negotiating reductions in nuclear weapons, getting the Soviets to change their behavior in places like Afghanistan, and "making sure Western Europe is safe against possible attack by Soviet and communist-bloc forces."

The results: 45 percent said reducing nuclear weapons should be the most important part of America's Soviet policy. Almost a third -- 31 percent -- said forcing changes in Soviet human rights policies should be this country's primary goal, and 18 percent said insuring the protection of Western Europe should be the main objective. Only 5 percent said pressuring the Soviets to change their behavior in places like Afghanistan should be the top priority.

Americans were somewhat cautious in assessing the impact of the proposed intermediate-range nuclear forces (INF) treaty that Gorbachev and President Reagan are expected to sign this week.

Almost half -- 48 percent -- said the treaty "won't have much effect" on the chances of a nuclear war between the United States and Russia. But another 40 percent said the agreement would decrease the chances of nuclear war; 9 percent said it would increase the chances, and 3 percent had no opinion.

Support for the treaty is broad, the survey disclosed. Slightly more than half of those surveyed -- 52 percent -- said they favored the agreement. Only 8 percent were opposed, and 40 percent had not yet made up their minds. Among those who had read or heard about the agreement, 61 percent favor it. And when those who said they had no opinion were asked which way they leaned, support among the whole sample jumped to more than 80 percent.

The poll showed that the public is somewhat optimistic that this week's summit will decrease tensions in the world.

More than three of five -- 63 percent -- said the summit will ease world tensions. But only 7 percent said it would ease tensions "a lot," while 56 percent said it would reduce tensions only some or a little.