When Secretary of State George P. Shultz sits down with Soviet Foreign Minister Andrei A. Gromyko Monday, much of Washington will focus on another high-level confrontation: the battle between Ronald Reagan, anticommunist, and Ronald Reagan, peacemaker.

According to many insiders, the latter private negotiations could prove decisive to the prospects for arms control in President Reagan's second term.

With senior aides divided into two groups on either side of the imaginary table, the politics of the second administration again will be colored by a struggle between moderates and hard-liners -- or, as some would say, naive dreamers and skeptical realists.

But such a portrayal is too simple to fit the facts. The arms control issues are more complex, the internal alliances more shifting and the outcome far more dependent on outside factors -- such as Soviet intentions and the stability of Soviet leadership -- than such a picture would suggest.

Yet the contrast between the Reagan who described the Soviet Union as "the Evil Empire" in 1983 and the Reagan who held out an olive branch at the United Nations last fall is real. So is the bitter split within the administration, exemplified by the curious phenomenon of every top arms control aide trooping off to Geneva with Shultz, as if each camp wanted to keep an eye on the other, even during a two-day preliminary session.

"Reagan would like to achieve the kind of success in arms reductions that would make him known as a peace president in a second term," one administration official in the "moderate-naive" school said.

Countered a senior official of the "hard-line-realist" camp: "The man is 73 years old. He is not going to change now. He is not going to accept a bad agreement."

During the past few weeks, the administration has succeeded in keeping many of its arms control deliberations secret, so any attempt to gauge the prospects for the future must be cautiously undertaken. Shultz may be taking more to Geneva, and back-channel discussions with the Soviets may have led to more agreement, than officials have disclosed.

But according to senior administration officials speaking both on and off the record, Reagan has decided one key issue in favor of the hard-liners: He will not bargain away his "vision" of a space-based missile defense system in exchange for reductions in Soviet offensive missiles. Several experts have said that such a trade would have offered the best hope for an agreement in the next four years.

"If we propose only a seminar in the advantages of defensive systems, we may not get talks at all, or they may not go very far," said Raymond Garthoff, who helped negotiate the SALT I treaty.

Beyond the decision on space-weapons research, which may represent Reagan's final decision or his initial bargaining stance, the second-term battle has hardly begun, according to most accounts.

The hard-line camp, represented by Defense Secretary Caspar W. Weinberger, appears to have won most of the skirmishes so far, even as the moderates, led by Shultz, have claimed success in "taking charge" of foreign policy.

If the two nations agree next week to resume substantive talks, more detailed internal negotiations will begin, with pressure for agreement coming from Congress and the European allies as well. At that time, the same difficult problems that helped derail arms control in the first Reagan administration, some of which have become even less tractable, will have to be faced again.

"Personally I don't think this administration is in terrific shape for these kinds of talks," Rep. Les Aspin (D-Wis.), newly elected chairman of the House Armed Services Committee and a key participant in the arms control debate of the first term, said in an interview. "It hasn't figured out where it stands on a number of issues."

Even a totally unified administration would be challenged by the problems:

* A new generation of mobile, easily hidden nuclear weapons, such as sea-based, low-flying cruise missiles or truck-mounted intercontinental missiles such as the U.S. Midgetman now being developed. These will be more difficult for intelligence satellites to find and, therefore, more difficult for treaties to limit.

* An increasing emphasis in both countries on the defensive arms that were limited by the 1972 Anti-Ballistic Missile Treaty, until recently considered one of the few successes of U.S.-Soviet negotiation. This emphasis has injected complicated issues into the talks and could prompt both sides to look for new offensive weapons to overcome prospective defense, some experts believe.

* Widely divergent views in the two nations of who is "ahead," whose weapons are more dangerous and who is the aggressor in world affairs. One result is that in the United States, and probably in the Soviet Union, there are few if any weapons systems that the military is prepared to forgo.

* The conviction in the Reagan administration, and probably in the Politburo, that the other side is untrustworthy and likely to use arms control talks only as a cover for building or retaining military superiority. The deep suspicion of Soviet intentions is shared by Reagan and his top advisers, including those who are considered "moderate."

"The problem is that the Soviets seek absolute security in a way that guarantees insecurity for everyone else," Shultz said in a recent speech, adding that they "can be expected periodically to do something abhorrent to us or threaten our interests."

The United States has 26,000 nuclear warheads, according to the respected Nuclear Weapons Databook, ranging from nuclear land mines to the destructive and accurate warheads of the Minuteman 3 missile. The Soviets' stockpile is between 21,000 and 42,000, according to Arms Control Today.

Officials in both countries acknowledge that such vast arsenals do not make sense. Beyond that agreement, however, they have not found a way to reduce their stockpiles while trusting their counterparts to do the same.

In an interview with The Washington Post, Shultz earlier attempted to explain how Reagan faced this apparent contradiction.

"What he found is that he can sit down with Gromyko, and both people can say -- Gromyko can say that the question of questions is, what are we going to do about these offensive nuclear weapons?" Shultz said. "And the president can say, I agree with you completely, got to do something about that.

"At that level of generality, there's always agreement," Shultz continued. "If you cast yourself back to the days when we were having our discussions in Geneva, at that level of generality or operational content -- however you want to put it -- there's disagreement.

"So what he is saying is kind of come in between, somehow, these levels and gradually move philosophic agreement down into operational detail," Shultz concluded.

The administration spent its first four years trying vainly to do just that. Whether it can do better in its second term, with few changes of personnel, is a matter of controversy here.

Several officials within the administration said they are better prepared to face the Soviets now. They said several officials -- Shultz, national security affairs adviser Robert C. McFarlane, negotiator Paul H. Nitze and Reagan himself -- had repositioned themselves to allow rapid and effective decision-making in a second term.

Nitze, an unyielding hawk through most of his career, earned a reputation as a "moderate" within the administration when he attempted, unsuccessfully, to fashion a compromise while representing Washington in talks on medium-range nuclear weapons. Reagan now has appointed him "special adviser" to Shultz, which places him above the harder-line negotiator Edward L. Rowny and was taken as a hopeful sign by arms control advocates.

McFarlane, the third man to serve as Reagan's national security affairs adviser, has taken control of the process more than either of his predecessors, Richard V. Allen and William P. Clark, according to observers. In the past few weeks, he has chaired several interagency meetings in preparation for the Geneva talks and has helped keep bureaucratic squabbling to a minimum, they said.

McFarlane's decision to accompany Shultz to Geneva affirms his role, according to one official, and assures the Soviets that Shultz can speak for Reagan and the government without having to return to Washington and tangle with Weinberger.

Shultz also has taken a more active role, officials said. When he replaced Alexander M. Haig, Shultz was a conservative economist with little knowledge of, or interest in, arms control, according to aides.

In the past two years, however, and particularly since Lebanon moved off the front burner as a U.S. policy issue, Shultz has devoted time and thought to arms control. A speech he gave in October, aides said, culminated a period of introspection about why it might be worthwhile to bargain with the Soviets even if they are, as Reagan said, "the focus of evil in the modern world."

"Some argue that if you cannot trust the Soviets, you should not negotiate with them," Shultz said then. "But the truth is, successful negotiations are not based on trust . . . . We need to make agreements that are trustworthy because both sides have incentives to keep them."

There is evidence to suggest that within the past few weeks the administration has maintained a back channel for communicating with the Soviets, which some observers take as a sign that Shultz is more in control. Officials had difficulty keeping a channel open during the first term, in part because of Soviet coldness but in part because so many factions insisted on being informed.

Shultz has had regular private meetings with Reagan, many of them devoted to discussions about "why you might want to negotiate with liars," as one aide put it. And Reagan, who was relatively disengaged from arms control disputes among his appointees in the first term and uninformed about aspects of the subject, also has become more involved, aides said.

In the past few weeks, Reagan has chaired five high-level meetings designed to thrash out positions for Geneva. His intervention in the "Star Wars" dispute -- deciding that research into missile defenses should not be a "bargaining chip" -- represents more personal involvement than he showed in most arms-talks decisions in his first years, officials said.

Moreover, most top aides agree that Reagan is committed to seeking an agreement with the Soviets. Urged on by his wife, Nancy, and top aide Michael K. Deaver, Reagan is sympathetic to the view that, having devoted himself to a U.S. military buildup in his first term, he is well-positioned to seek a peace treaty in his second.

But other officials argued that too much should not be read into any of those signs. For example, they said that Nitze, although appointed special adviser, had taken himself out of consideration for the job of negotiator in future talks, perhaps leaving more room for Rowny.

In addition, they noted that the "moderate" faction had not managed to displace Richard N. Perle, the leading advocate within the administration of the "skeptical" view.

Perle, assistant secretary of defense for international security policy, is going to Geneva, too. Though he has been cut out of some of the decision-making leading up to the conference, officials said, Weinberger is likely to make sure that he plays an active role in formulating positions if substantive talks resume.

Weinberger, an ally of Reagan since Reagan was California governor, has been counted out before in internal disputes, on foreign policy and budget questions, but has come out on top more often than not. In a recent interview, he dismissed suggestions that others in the administration wanted him to take a lesser role in arms control.

"Oh, I think Tass went much further than that," he said. "They wanted me fired."

Weinberger said he favored "arms reductions" and the negotiating process but disliked past agreements because they allowed "more expansion" of arsenals.

"The Soviets always have an advantage in dealing with impatient democracies, because the results are always eagerly awaited and measured by whether you get agreements or not, without too much regard, sometimes, for what's in the agreements," Weinberger said. "But I think the president is very aware of that set of problems."

Perhaps most significant is that every decision, so far, appears to have gone the Defense Department's way. There will be no early moratorium on testing antisatellite weapons, as some State Department officials wanted, and defensive systems will not be discussed in the same forum as offensive, which would have made direct trading easier.

In addition, most officials express the belief that the United States will resume talks by offering the same proposal for reducing strategic weapons that was on the table when the Soviets walked out in 1983, a tactic conforming to Defense Department wishes. Pentagon officials believe that because the Soviets walked out, they should make the first step toward compromise.

When Reagan made his first conciliatory speech to the Soviet Union one year ago, many Democrats scoffed that he was only engaging in an election-year detente. Shultz has said, however, that the 1984 Reagan still lives.

"Actually, the president seems to be going out of his way to say that the circumstances haven't changed now that the election is over," he said. "He's making a point that he is essentially taking the same positions that he was taking before."

A former senior Republican official who still has access to the administration disagreed. Reagan, this observer said, will never overcome the resistance within his administration and his own longstanding suspicion of the Communists to sign a balanced agreement.

"Reagan wants to be peacemaker like Deaver and Nancy have told him, but he doesn't understand what that takes," the former official said. "When it comes to an actual agreement that will leave the Soviets with some advantages, he'll balk."

Arms control during the next few years may, in fact, hinge on other factors -- Soviet behavior internationally, presidential politics here, the health of President Konstantin U. Chernenko. But Paul C. Warnke, who negotiated the still unratified SALT II treaty, said Reagan's final disposition could be key.

"We'll find out if he's a great communicator but a poor leader," said Warnke, a Democrat. "Leaders don't leave it to be fought out among assistant secretaries. Leaders have to make choices."