Slightly more than half of all Americans approve of the proposed treaty banning medium- and shorter-range nuclear missiles, but an even larger majority acknowledges they know little or nothing about it, according to a new Washington Post-ABC News poll.

The survey showed that 52 percent of the 1,007 persons interviewed said they supported the intermediate-range nuclear forces (INF) treaty. Only 8 percent were opposed, and 40 percent said they did not know enough about the agreement to have an opinion.

Other results suggest that even those Americans with no firm opinion are predisposed to favor the treaty. When respondents who said they were undecided were asked which way they were leaning, support for the pact increased to 82 percent.

The survey seemed to describe an American public eager to embrace the proposed INF treaty with the Soviets. But some results also suggest that opinion on the treaty, while generally positive, is also largely uninformed and easily led, factors that could be exploited by treaty opponents during the ratification debate in the Senate next year.

The survey disclosed that half of all respondents said they knew "not much" about the treaty and another 22 percent said they knew "nothing at all." Only 46 percent mentioned Europe when asked if they knew where the U.S. missiles to be eliminated by the agreement were located.

The survey also found that public expectations are high for next week's summit between President Reagan and Soviet leader Mikhail Gorbachev. Nearly three of five said next week's summit "will lead to important accomplishments in arms control." That contrasts sharply with a Post-ABC survey taken before the 1986 summit in Iceland, where 40 percent of those interviewed said they expected significant results from that meeting.

Americans also expect that a signed treaty next week will lead to further arms agreements, a view held by 74 percent of those questioned. And almost half said they favored additional major arms control agreements with the Soviets.

Two-thirds of those surveyed also were willing to sacrifice Reagan's goal of developing space-based defenses against nuclear attack -- the "Star Wars" plan -- for an agreement to substantially reduce nuclear weapons.

Americans continued to be impressed with Gorbachev, according to the poll. Almost three out of five respondents -- 59 percent -- said they had a favorable impression of the Soviet leader. In comparison, 63 percent gave Reagan a favorable rating.

The poll shows that the INF treaty to be signed Tuesday appears to be most vulnerable on an issue that has scuttled previous U.S. and Soviet arms control efforts: fear that Moscow will cheat.

Nearly two out of three respondents -- 65 percent -- said they believed that the Soviets would try to cheat on a nuclear arms agreement. And 70 percent said that suspicions that the Soviets would attempt to cheat is "an important enough reason in itself" to reject the treaty.

Fears of Soviet cheating on an arms agreement have declined in the past two years. The poll showed that 65 percent believed the Soviet Union would attempt to cheat on the agreement to gain an advantage over the United States. That is an 11-point decline since a Post-ABC poll in November 1985.

The treaty is somewhat less vulnerable to arguments that it would increase the chances of a Soviet invasion of Europe, according to the poll.

Respondents were asked, "Suppose there was a chance that removing medium-range missiles from Europe and the Soviet Union would make a Soviet invasion of Western Euorpe more likely. Would that be an important enough reason in itself for the U.S. to reject the medium-range missile agreement or not?"

Slightly more than seven of 10 said it was. But when asked if they believed that the proposed agreement would make such an invasion more or less likely, 15 percent said more likely, 10 percent less likely and 35 percent said it would make no difference, with 40 percent undecided.

The survey disclosed that support for the proposed treaty was broadly based. But slight differences did emerge. Approval was, for example, somewhat higher in the East and West than in the Midwest or South.

Republicans were more supportive of the treaty than were Democrats by 56 to 48 percent. And the difference between liberals and conservatives was similarly small: 58 percent of the self-described liberals said they favored the treaty, as did 52 percent of the conservatives.

Nearly two out of three respondents said both countries would benefit equally from the proposed arms agreement. Ten percent said the United States would benefit more, and 23 percent said the Soviets would.

The summit and proposed arms agreement have produced mixed political benefits for Reagan, the poll shows.

A majority of the public -- 56 percent -- now approves of the way the president is handling nuclear weapons policy, a 9-point increase since March. And Reagan's standing as a peacemaker improved even more: 83 percent of those surveyed said he "seriously wants to make progress in arms control." In June,

67 percent expressed a similar view.

The public remains skeptical, however, about Reagan's willingness to make the compromises to achieve an arms control agreement. Nearly two-thirds said he "will not go far enough" to win an arms pact, while 23 percent said he would "go too far."

In the poll, the margin of sampling error for overall results is plus or minus 3 percentage points. Results based on a portion of the sample have a somewhat larger error margin. Sampling error is only one of many potential sources of error in any public opinion poll.

Polling analyst Kenneth E. John contributed to this report.