President Reagan said yesterday that his Strategic Defense Initiative (SDI) emerged intact from his summit with Soviet leader Mikhail Gorbachev despite Soviet objections to the missile-defense plan, and he contended that the superpowers are "in a better position to make tangible gains in arms reduction than at any time in the last 40 years."

Reagan, in an upbeat mood after three days of meetings with Gorbachev, won plaudits from a bipartisan group of congressional leaders who gathered in the Cabinet room of the White House to hear his report on the summit. When the president entered the room, the leaders gave him the first standing ovation he has received at such a meeting in the past four years.

"What happened?" Reagan said, when the applause died down. "Did you just see one of my old movies?"

White House officials and Reagan strategists claimed the summit had given the president a much-needed political shot in the arm. Some prominent Democrats agreed.

Sen. Alan Cranston (D-Calif.), an arms-control advocate often critical of Reagan policies, said "there has to be some lasting benefits" for the president because he may now have a chance to achieve an agreement slashing the superpowers' strategic nuclear arsenals. Senate Majority Leader Robert C. Byrd (D-W.Va.), who emphasized his support of SDI, said of the summit, "It helps the president, no question. It should."

But conflicting reports emerged yesterday over an exchange between Reagan and Gorbachev regarding Soviet military support for the Marxist Sandinista government of Nicaragua and U.S. military aid to the contra rebels there.

After their session with Reagan, some lawmakers said they understood Gorbachev had offered to suspend shipments of heavy arms to Nicaragua as long as the Central American peace process is going forward and the United States does not resume arms shipments to the contras.

However, enthusiastic White House officials suggested that this third summit between the two leaders would prolong the effective life of Reagan's presidency, especially because he has agreed to go to Moscow during the first six months of next year, where a strategic arms reduction agreement could be signed. The joint statement issued by the two leaders commits Reagan to this meeting whether or not an agreement is ready to be signed.

According to White House chief of staff Howard H. Baker Jr., Reagan "is going to be an active and assertive president longer than most in the final year."

The joint statement sidesteps the crucial issue of limiting SDI research, and Gorbachev made it clear in a news conference before he left Washington Thursday night that he remains as much opposed as he always has been to Reagan's plan for a missile defense system.

But when asked at a news conference whether the SDI issue had merely been postponed, the president said: "No, it resolves it." He said, "We have agreed that we are going forward with whatever is necessary in the research and development {of SDI} without regard to an interpretation" of the 1972 Antiballistic Missile (ABM) Treaty.

Reagan said the main threat to continuing the so-called "Star Wars" research program comes not from the Soviets but from Congress, which has cut funding and prohibited any tests that violate a strict, or limited, interpretation of the ABM Treaty. The administration favors a broad interpretation that would allow expanded SDI testing and development.

A White House official who declined to be identified acknowledged yesterday that Congress' action confining Reagan to a narrow interpretation of the treaty may have made possible the summit accommodation on SDI. Soviet Foreign Ministry spokesman Gennadi Gerasimov took a similar position before the summit, saying that passage of the bill restricting SDI tests and Reagan's signing of it had made the U.S. position more acceptable.

By avoiding a conflict on the missile defense issue, which led to the collapse of the 1986 superpower summit in Reykjavik, the two sides may have eased negotiations for an agreement to make 50 percent cuts in strategic nuclear arms. However, Reagan and Gorbachev have said that much work must be done before such an accord can be signed.

White House optimism about the summit's impact on Reagan's political fortunes was bolstered by a survey taken by the president's pollster, Richard B. Wirthlin, which showed a 61 percent public approval rating for the president. It was the first time that Reagan had received an approval rating above 60 percent since the disclosure a year ago of U.S. arms sales to Iran.

Yesterday, politicians from both sides of the aisle credited the president's performance. Asst. Senate Minority Leader Alan K. Simpson (R-Wyo.) said the leadership meeting was the most conciliatory and congenial he has ever attended -- "a spirit I have never seen before."

House Speaker Jim Wright (D-Tex.), often at odds with the president, praised Reagan during the leadership meeting, according to participants. Wright, on the White House driveway afterward, also lauded the Soviet leader, saying, "If Gorbachev were an American, he'd be as American as apple pie."

Some senators warned that Americans should not exaggerate the benefits of the talks. Sen. Sam Nunn (D-Ga.) called it "a good summit," but said the Soviets should be judged by deeds, not words, "and we haven't seen the deeds, not yet."

Sen. Steve Symms (R-Idaho) said, "Right now there's euphoria, but after awhile people will realize they're still killing people in Afghanistan."

These criticisms were dismissed at the White House. Said communications director Thomas C. Griscom, "This president is doing what he wanted to do from the beginning -- get some movement on nuclear arms."

In his answers at a news conference for non-Washington-based reporters, Reagan said "the people of both countries" were the summit winners. He expressed respect for Gorbachev, saying that while he and the Soviet leader disagree on fundamental issues, "there's a certain chemistry between us."

The president also praised "the unity and responsibility demonstrated by the alliance" leading up to the summit and telephoned West German Chancellor Helmut Kohl, Japanese Prime Minister Noboru Takeshita and British Prime Minister Margaret Thatcher to thank them for their support.

Staff writers Helen Dewar and David Hoffman contributed to this report.