For President Reagan, next week's summit with Soviet leader Mikhail Gorbachev offers an opportunity to realize a dream of reducing the superpowers' nuclear arsenals by implementing an idea that has been in Reagan's head for years.

But the dream is a nightmare for ardent conservatives who now fear that Reagan has betrayed them.

"There are certain things that certain presidents can do," said White House communications director Thomas C. Griscom, who compared Reagan's dealings with Gorbachev to President Richard M. Nixon's opening to China. "This is the right president at the right time to get an arms-control agreement."

Some conservatives view Reagan's determination to sign one arms-reduction treaty and push ahead on another as a transparent political attempt to secure a favorable legacy. But longtime advisers and friends of the president say he has nurtured the idea of nuclear arms reduction for at least seven years and is not going to be diverted from his goal even by those who consider themselves his core constituents.

Martin Anderson, a conservative economist who served as a Reagan adviser during three presidential campaigns, recalled that Reagan surprised his strategists in 1980 by coming up with an idea "far more radical than a nuclear freeze," a favorite liberal idea in the early 1980s. Reagan's notion was that both sides were so strong they could afford to make large-scale reductions in their nuclear arsenals without damaging their security -- and that the Soviets might be persuaded to do this if confronted with a continuing U.S. military buildup.

"Aides kind of humored him about this, which as best as I can determine was Reagan's idea," Anderson said.

What emerged from the campaign discussions was a Reagan view that the United States should go all-out in a military buildup designed to force the Soviets to the bargaining table. Meeting with reporters and editors of The Washington Post on June 19, 1980, candidate Reagan predicted that the Soviets would then negotiate because they couldn't afford to match U.S. military expenditures.

"They've diverted so much to {the} military that they can't provide for consumer needs," Reagan said.

Reagan also welcomed the idea of negotiating with the Soviets, provided he was doing so from a position of strength. He prided himself on the negotiating abilities he had displayed as president of the Screen Actors Guild and said frequently that he was willing to sit down for "as long as it takes" to negotiate a treaty to reduce arms, not just control the arms race -- the fatal flaw, he argued, in the SALT II treaty President Jimmy Carter negotiated.

After Reagan became president, he launched a military buildup whose cost exceeded even his campaign promises. But negotiations with the Soviets proceeded slowly during Reagan's first term.

Reagan blamed this on a succession of ailing leaders in the Kremlin, saying he wanted to negotiate but that the Soviets "kept dying on me." The president's critics and even some administration officials attributed the lack of progress to wrenching debates within the U.S. government, where Pentagon hard-liners opposed any arms deal with the Soviets.

The president, while eager to meet with a Soviet leader, was unable or unwilling to come down firmly on one side of the dispute between the rival power centers inside his administration.

The dynamics of this stalemate were changed by events in both the United States and the Soviet Union. After coming to power in 1985, Gorbachev displayed a willingness to negotiate sweeping arms reductions that distinguished him from his predecessors. Within the Reagan administration the power of hard-liners waned steadily until last month, when the resignation of Defense Secretary Caspar W. Weinberger and his replacement by Frank C. Carlucci removed the last determined hard-liner from the upper reaches of the government. To emphasize the point, Carlucci replaced Pentagon arms specialist Frank J. Gaffney Jr., the most visible remaining symbol of U.S. skepticism over superpower arms deals.

Reagan came back from his first summit with Gorbachev in Geneva in 1985 convinced that he was a different kind of Soviet leader, one with whom he could negotiate. In the euphoria of post-summit evaluations, the president compared the Soviet leader to movie producers he had bargained with in Hollywood.

The president's belief that Gorbachev was different persisted even after the disappointment of last year's Reykjavik, Iceland, summit in which the two leaders discussed a breathtaking array of proposals, including destroying their nuclear arsenals, without reaching any agreements.

Last Sept. 22, at a White House meeting arranged by Paul M. Weyrich of the Free Congress Foundation, Reagan startled a group of conservative leaders with his positive evaluation of Gorbachev.

As Weyrich recalled it, the president began his remarks by saying that they "should know better" than to think that his views on communism had changed. However, Weyrich said, the president went on to say that "Gorbachev is a different kind of Soviet leader, the first to say that his goal is not conquering the West . . . . Reagan's view was that internal conditions in the Soviet Union were forcing Gorbachev to deal differently with the West."

The conservatives pressed their case that the Soviets remained the "Evil Empire" that Reagan had labeled them and cited examples of Soviet conduct in Afghanistan and other regional conflicts. Reagan did not take issue with these points, but he did not change his opinion of Gorbachev, either.

"The president feels we have simply misunderstood everything," Weyrich said in an interview last week.

Reagan has not convinced the conservatives of the merits of the intermediate-range nuclear forces (INF) treaty that he and Gorbachev are scheduled to sign Dec. 8, let alone of the benefits of proceeding with negotiations that could lead to deep cuts in the strategic arsenals of the two superpowers -- the intercontinental missiles and bombers.

Some White House officials are now talking as if such a treaty could be ready to sign at a Moscow summit early next year. Others say that even if election-year pressures and a fight over ratification of the INF treaty make this impossible, U.S. and Soviet negotiators may make enough progress that Reagan would become a supporter of a strategic treaty in the next presidency, no matter which party is in the White House.

But the feeling that Reagan is betraying basic principles is running high among conservatives, provoking unusual personal criticism of the president of a kind that in the past has usually been limited to attacks on White House aides or Secretary of State George P. Shultz.

"The alarm bells are ringing all over the place," said Sen. Richard G. Lugar (R-Ind.), an INF treaty supporter. "You'd have to be blind and deaf not to see and hear the problems."

Lugar, warning of Senate reservations that might effectively kill the treaty, said opponents "feel they have to make a federal case out of INF to avoid going further on arms control."

The most intense public criticism of Reagan's behavior comes from unelected conservative activists such as Weyrich and Richard Viguerie, who have been frequent critics of administration policies.

Weyrich called Reagan "a weakened president, weakened in spirit as well as clout, and not in a position to make judgments about Gorbachev at this time." Viguerie said signing of the INF treaty will represent "a splitting of the blanket -- conservatives will file for divorce and never reconcile again."

Privately, some elected conservatives are almost as critical of Reagan. One prominent congressional Republican, speaking on condition that he not be identified, said, "There is a sense that Reagan is not leading the charge anymore, that he has forgotten the basic principles on which he is elected."

Sen. Steve Symms (R-Idaho), a conservative carried into office in the Reagan landslide of 1980, last week voiced a concern often expressed in the past by Reagan -- that the Soviets will not comply with any agreements they sign, no matter what the verification procedures. "Peace and freedom are inseparable, as the president used to say," Symms said. "I'm concerned that we'll end up keeping the peace and losing the freedom."

The president's friends and advisers say these conservatives heard only the part of the Reagan message with which they agreed -- they liked the buildup, but not the ultimate goal of negotiations.

White House officials say they think Reagan can win the argument because he has the support of an overwhelming majority of the American people in dealing with the Soviets.

"He isn't going to convert the right-wingers, but he can go over their heads and speak directly to their constituency," a White House official said.

Another senior official said Reagan has expanded the GOP base so that "the right-wingers" are a smaller and less influential part of the whole than when he was elected. In any case, the official said, polls taken for the White House show that a majority of conservative voters support the treaty.

"If you say no treaty with the Soviets, that's not where the public is," the official said. "It's a different party now" than in 1976, when Reagan led a campaign against the Panama Canal treaties.

A new survey taken by the Democratic firm Marttila and Kiley supports this view. The poll found 74 percent of Republicans and 69 percent of Democrats favorable to the INF treaty.

"At the same time, voters regard the military buildup that has marked the Reagan years as a public mandate," pollster Thomas Kiley found. "By a consensus-level margin of 69 percent to 27 percent, they believe that this buildup has been necessary."

This prevailing sentiment is unlikely to repair the shattered relationship between Reagan and conservative activists who gloomily predict that an era of good feeling between the United States and the Soviet Union inevitably will lead to reduced defense spending, the emasculation of the Strategic Defense Initiative and the abandonment of the Nicaraguan contras and other anti-Soviet "freedom fighters" in regional conflicts.

"The great conservative dream was that Ronald Reagan, in his last two years, not having to worry about the election or any further aspirations, would set the stage for the conservative revolution," said Weyrich. "On the contrary, we have Ronald Reagan who, freed from all constraints, is endangering what he has already accomplished and behaving in a way that will have a harmful effect on the future. It's ironic."Staff writers David Hoffman and Helen Dewar contributed to this report.