President Reagan has won immediate, but perhaps temporary, political gains from last week's summit and reversed sagging public confidence in his administration, according to a new Washington Post-ABC News Poll.

The most recent survey also disclosed increased support for the treaty to eliminate medium- and shorter-range nuclear missiles in Europe, which was signed last week by Reagan and Soviet leader Mikhail Gorbachev and now goes to the Senate for ratification.

The poll also found that slightly more than half of those interviewed opposed scrapping Reagan's Strategic Defense Initiative, the space-based missile defense plan, to win additional arms reductions from the Soviets.

In another finding, the survey revealed that more Americans have a favorable view of Gorbachev than have a favorable view of Reagan. Nearly two-thirds of those interviewed -- 65 percent -- said they had a favorable impression of Gorbachev, while 61 percent said they had a favorable view of Reagan.

Americans, however, viewed Reagan as more effective than Gorbachev at the summit talks: 42 percent of those surveyed said Reagan did the better job, while 26 percent said Gorbachev.

Following a week of high-powered summitry, the Reagan presidency emerged as the clear early winner. The evidence:Reagan's overall job approval rating surged 8 percentage points to 58 percent in less than two weeks, the highest it has been since September 1986, before the Iran-contra affair unfolded. Almost three out of five -- 57 percent -- said they approved of the way Reagan was handling foreign affairs, his best showing in 15 months and up 11 percentage points in less than two weeks. More than three of four -- 77 percent -- said they approved of the way Reagan is handling relations with the Soviets, up 11 points since the presummit poll and the best rating of his presidency.

The poll also showed that nearly half -- 49 percent -- of those surveyed said the country is generally going in the right direction, up from 35 percent in the pre-summit survey.

Those gains will likely fade as other problems force their way back onto the public agenda. In fact, America's post-summit euphoria has done little, if anything, to diminish concerns about the economy: Almost half of those surveyed -- 49 percent -- said the economy is getting worse, compared with 53 percent in the pre-summit poll Nov. 30 to Dec. 2. And nearly half said in the most recent survey that the stock market plunge "means the United States is going into an economic downturn."

On the Intermediate-Range Nuclear Forces Treaty signed last week, slightly more than three out of five Americans -- 62 percent -- now said they favored it, up 10 percentage points from a Post-ABC survey conducted before the summit. When those who said they had no firm opinion were asked which way they were leaning, support rose to 82 percent. Only 6 percent now oppose the treaty.

The public's generally high expectations for the summit appear to have been met, even exceeded, the survey showed. More than three out of five -- 62 percent -- said the summit accomplished more than they first thought it would. Twenty-four percent said Reagan and Gorbachev accomplished less, and the remainder were undecided.

A big majority of Americans also hopes the treaty leads to substantive reductions in both nations' arsenal of more powerful -- and more threatening -- strategic nuclear weapons. Sixty-one percent said they favored the United States signing a treaty with the Soviet Union that would cut strategic nuclear weapons by half, according to the poll.

In fact, almost seven of 10 -- 68 percent -- said Washington and Moscow ultimately should agree to eliminate all nuclear weapons. But only slightly more than half said they expected total disarmament would "make the world more safe," while almost one of five said it would make it less safe and about one of four said it would make no difference.

Other results suggest that treaty opponents, particularly those who aspire to the presidency, oppose it at their risk. The agreement enjoys generally strong support from Republicans and Democrats, liberals and conservatives, and men and women. And when asked whether they would be more or less likely to support a presidential candidate who opposed the treaty, 43 percent said less likely. Only 12 percent said they would be more likely, while 41 percent said it would make little difference.

The survey also showed that large numbers of Americans remain undecided about the Strategic Defense Initiative, the ambitious and controversial plan for a shield against nuclear attack. More than two of five -- 43 percent -- said they had no opinion about SDI, while 39 percent said they favored it and 18 percent were opposed.

Seven out of 10 respondents did say they supported the overall goal of reductions in nuclear weapons over Reagan's SDI. But 53 percent reject abandoning SDI, even if it is the only way to get further arms reductions with the Soviets, while 40 percent would give up the plan for weapons cuts.

The survey was conducted Dec. 11 to 13. A total of 1,007 adults nationwide were interviewed by telephone for this poll. Margin of sampling error for the overall results is plus or minus three percentage points. Samping error is, however, only one of many potential sources of error in this or any other public opinion poll.

Polling analyst Kenneth E. John contributed to this report.