President Reagan told conservative supporters yesterday that it was time for the 115,000 Soviet troops in Afghanistan to "pack up, pull out and go home" and said he would press the issue at summit discussions next week with Soviet leader Mikhail Gorbachev.

Saying that even the Soviets recognized that the "puppet" regime they support in Kabul is "discredited and doomed," Reagan called on Soviet leaders to "bite this bullet" and negotiate with the Afghan resistance on a political settlement of the conflict.

"It's time they set a date certain for the complete withdrawal of all troops from Afghanistan," Reagan said.

Reagan made this declaration in a speech to the Heritage Foundation, which has provided many ideas and appointees for his administration, on a day when various White House and State Department officials spelled out U.S. positions and expectations for the Washington summit.

Meanwhile, Senate Minority Leader Robert J. Dole (R-Kan.) disclosed that the Soviets had ruled out a visit to Capitol Hill by Gorbachev. Dole said he had been told by Secretary of State George P. Shultz that the Soviets were "a little upset" that objections from congressional conservatives had forced the collapse of a plan to have Gorbachev address a joint meeting of Congress.

A senior White House official said that the U.S. and Soviet governments had worked out a compromise under which nine congressional leaders would meet with Gorbachev at the Soviet Embassy the morning of Dec. 9. The plan has been accepted by the congressional leadership, the official said.

Reagan was greeted by total silence during his speech when he asked support for the intermediate-range nuclear forces (INF) treaty that he and Gorbachev are scheduled to sign on Dec. 8. But the audience applauded when the president vowed once again that he would refuse to bargain away his missile defense plan, the Strategic Defense Initiative. There was more applause when Reagan called upon the Soviets to withdraw from Afghanistan after eight years of occupation.

Later in the day, Undersecretary of State Michael H. Armacost told reporters at the State Department that it was his "impression" that the Soviets were no longer talking a timetable for withdrawal from Afghanistan that is contingent upon formation of an interim government.

"That's a fairly important point, if true, because it's always possible to string out discussions of interim government arrangements indefinitely and thereby avoid commencing the withdrawal," he said. "It's the completion of withdrawal that's important."

Gorbachev said in an interview last night with NBC's Tom Brokaw that "if the American administration really does sincerely want" a political solution to the Afghanistan situation "it could be done very quickly." The Soviets have demanded that the United States stop providing arms to the Afghan rebels as part of any settlement.

Armacost said that cessation of U.S. aid to the Afghan resistance depends on Soviet willingness to specify a withdrawal date. He said this has been "the sticking point" for many months in U.S.-Soviet talks about an Afghanistan settlement.

In his speech to the Heritage Foundation, the president restated his commitment to SDI, calling it "a cornerstone of our security strategy for the 1990s and beyond," and vowing, "We will research it, we will develop it, and when it's ready, we will deploy it."

Reagan called upon the Soviets to "stop holding strategic offensive missile reductions hostage to measures that would cripple our research and development of SDI" and said the Soviets have spent $200 billion to develop and deploy an antiballistic missile system of their own.

The president said that this "Red Shield program and the construction of the Krasnoyarsk radar as part of an early warning and tracking system" could mean that the Soviets "may be working toward a breakout" from the 1972 Antiballistic Missile (ABM) Treaty.

Chief U.S. arms negotiator Max M. Kampelman, in a briefing for reporters, went out of his way to discourage any suggestion that the administration is seriously considering any shift in its existing negotiating position on SDI.

Kampelman, however, noted that since the beginning of the Nuclear and Space Talks in Geneva in March 1985, "there has been a gradual change in the Soviet position, one that we have noted with interest, and which seems to take the form of seeking some notion of predictability" in the relationship of strategic defense to strategic offense.

"We are certainly prepared to join in an exploration of how we can have predictability in both of our interests. And if we can move to serious discussions in that area, I think they can be fruitful," Kampelman said. He indicated, as did Shultz in a weekend television interview, that the United States is interested in enhancing predictability and stability through an agreement with the Soviets not to withdraw from the ABM Treaty for an extended period.

If a nonwithdrawal period were worked out, the problem remaining would be how to interpret the constraints of the treaty. On this issue, the Soviets have proposed to negotiate with precision those items that could be sent into space.

Kampelman said accepting the Soviet proposal would be like getting into "kind of a swamp." But he added, "Any proposal they come in with, we will seriously examine and consider, without any doubt."

Regarding a strategic arms treaty mandating sharp cuts of up to 50 percent in U.S. and Soviet long-range nuclear arsenals, Kampelman said important decisions should be made soon if the objective of treaty completion in the first half of next year is to be met.

One objective of the Washington summit, as Kampelman related it, is to resolve the issue of specific ceilings on long-range ballistic missiles and other categories of long-range weapons. This is known among arms control specialists as the "sublimits" issue and has been the subject of several new Soviet proposals and suggestions in recent weeks, suggesting to some U.S. officials that it is ripe for resolution.

A senior State Department official who briefed reporters at the White House said human rights issues will "figure very prominently" in the summit discussions but that, at this point, no dramatic breakthroughs are anticipated. The official, who spoke to reporters on condition he not be identified, said the emigration of Jews from the Soviet Union, an issue of particular sensitivity in the United States, "appears to be on an official plateau" of 700 to 900 per month, much more than last year but nowhere near the "tens, almost certainly hundreds of thousands that would like to leave."

Reagan, in his Heritage Foundation address, called on the Soviet Union to demonstrate its "sincerity for reform" on religious matters by legalizing the Ukrainian Catholic Church.

Reagan called glasnost, Gorbachev's much-discussed policy of greater openness, "a promise unfulfilled."

Assistant Secretary of State Rozanne L. Ridgway, who also briefed reporters, said that Reagan, as host, will open the first substantive meeting of the summit next Tuesday morning by offering the floor to Gorbachev. She said she expects Gorbachev to begin with "a broad review of the relationship."

Tuesday's talks, in the morning and afternoon, will probably be devoted mainly to arms control issues and human rights questions, Ridgway said. Sometime on the first day, working groups of high-level aides will probably be established on arms control and perhaps other subjects, she added.

With the aides at work elsewhere, the two leaders are likely to go on Wednesday morning to discussions of regional issues and bilateral questions. In the final two meetings, on Wednesday afternoon and Thursday morning, she said, the two leaders will probably go back to arms control with the benefit of their aides' work.

White House spokesman Marlin Fitzwater said Gorbachev is expected to hold a news conference on Thursday afternoon after his final departure from the White House, and that Reagan plans to address the nation via television Thursday evening.

Staff writers Helen Dewar and David B. Ottaway contributed to this report.