Soviet leader Mikhail Gorbachev arrives this afternoon to meet President Reagan at a Washington summit that high-ranking administration officials say holds the promise of a new era in U.S.-Soviet relations.

While emphasizing that the success of the two leaders' third summit is not guaranteed, these officials said it could develop momentum for an agreement on deep cuts in the strategic nuclear arsenals of the rival superpowers and also ease the way toward resolving the war in Afghanistan and other regional conflicts, human rights and economic issues.

A highly successful summit could lead to fundamental gains in relations between the leading nations of the two post-World War II military and political blocs, perhaps bringing major changes in the global scene and in the internal policies and politics of both countries. In the United States, a basic improvement in U.S.-Soviet relations could spur political realignments, especially to the disadvantage of the right-wing Republican constituency, which until now has been Reagan's most committed and loyal base of support.

U.S. officials, speaking for an administration that appears to be reaching a rare degree of consensus on international affairs after nearly seven years in office, expressed particular optimism about the possibility of progress on strategic nuclear arms. Reagan and Gorbachev have said they would like to sign a treaty cutting strategic arsenals by 50 percent at a Moscow summit next year.

A senior official said the aim of this week's summit is "to settle as much as you can and simplify things so that the two leaders can issue instructions to negotiators . . . so that you can get a treaty in the first half of 1988."

White House chief of staff Howard H. Baker Jr. said in an interview it is "conceivable, although it would be difficult" to complete a strategic arms treaty in time to gain Senate ratification next year despite political obstacles facing any accord in a presidential election year. Another White House official said political hurdles "may be surmountable" if both parties nominate candidates committed to strategic arms reduction.

Secretary of State George P. Shultz in an interview said the United States will push for progress on its broad agenda of human rights, arms control, regional issues and bilateral accords if the Washington summit ends on a positive note.

Shultz said there is "a sense of a certain amount of motion" on regional conflicts such as those in Afghanistan, Cambodia and southern Africa "that have been on dead center for quite a while." He said the administration in its final year will seek to resolve as many Third World disputes as possible, in the past major impediments to long-term improvement in U.S.-Soviet relations.

Deep cuts in U.S. and Soviet strategic nuclear arsenals could give Washington and Moscow greater leverage to oppose the acquisition of atomic weapons by other nations. Such cuts could also provide a solid basis for the talks on reductions in conventional (nonnuclear) forces from the Atlantic to the Urals that are expected to begin between NATO and the Warsaw Pact nations next spring or summer. Reductions in conventional armies, more than cuts in nuclear weapons, could bring savings in the military budgets of the superpowers and their allies.

If the Soviet Union should withdraw its troops from Afghanistan, as public statements by Gorbachev and others increasingly suggest will occur, this likely would bring immediate, extensive improvement in Soviet relations with the Islamic world, according to Undersecretary of State Michael H. Armacost.

Together with resolution of the war in Cambodia, which showed movement diplomatically for the first time last week in a meeting in France involving the former Cambodian head of state, Prince Norodom Sihanouk, this could fundamentally improve Soviet relations with China and other powers of the increasingly important region of East Asia, Armacost said.

As the capital braces for Gorbachev's arrival, none of these positive developments is a certainty. Even a slow, steady improvement in Soviet policy and U.S.-Soviet relations, which many officials consider more likely and more desirable than rapid gains, remains a question that may only begin to be answered with the announcement Thursday afternoon of the results of this round of Reagan-Gorbachev talks. Nevertheless, various hints from Gorbachev and his aides of determination to move ahead and a certain flexibility on key issues has raised the possibility that the summit may be a clear-cut success.

A White House official said it is significant that the intermediate-range nuclear forces (INF) treaty that will eliminate medium-range and shorter-range nuclear missiles will be signed on Tuesday, the first day of the summit, after the first of five scheduled meetings between the two leaders.

"The symbolism of this is that you have completed a first step rather than capped the summit and that you must do more to make the summit a success," the official said.

The INF treaty was declared complete yesterday by Shultz, who said all that remains to be done is to inscribe the accord on special treaty paper in preparation for the White House signing.

U.S. sources said final agreement was reached late Saturday after a last-minute dispute was resolved through a hastily arranged exchange of messages between Shultz and Soviet Foreign Minister Eduard Shevardnadze.

The previously unreported dispute, sources said, was over where U.S. teams would enter East Germany to inspect missile sites being eliminated under the treaty. Washington refused a Soviet and East German proposal that the inspectors enter via an airport outside Berlin, because of the potential effect on complex East-West relationships in that divided city. The Soviets finally agreed to Leipzig, 113 miles southwest of Berlin, as the entry point.

Whether the summit develops forward momentum is likely to depend on what happens in the eight hours of scheduled talks. About a third of this time is expected to be spent in intimate conversation between Reagan and Gorbachev with only interpreters and bilingual notetakers present, U.S. officials said.

"There are sure to be some surprises this time," said a senior official, harking back to the unexpected developments at the summit last October in Reykjavik, Iceland. "Gorbachev and other Soviet officials have come to every high-level meeting with proposals that have gone well beyond their previous positions."

Reagan, who prides himself on his negotiating skills, is prepared to deal seriously with any Gorbachev proposal and he is also prepared to press the Soviet leader on regional and human rights issues, officials said.

"The president believes he can talk to this Soviet leader," said a senior White House official. "He feels they have a relationship. The schedule has enough built-in flexibility that you don't have to start taking things apart if a meeting runs over or the two leaders want another session."

In the run-up to the summit, Reagan has mixed hard-edged rhetoric about Soviet conduct in Afghanistan and Soviet human rights practices with frequent expressions of respect for Gorbachev and the changes he is attempting to accomplish in the Soviet Union.

Stuart K. Spencer, a longtime friend and political adviser who met with the president Friday, said Reagan's statements reflect a negotiating strategy rather than a desire to soothe the feelings of conservatives skeptical of U.S.-Soviet arms control treaties.

"The idea that Reagan is saying all these things to appease the right-wingers is baloney," said Spencer. "That's his way of negotiating with the Soviets. He came into office with the goal of having this meeting. He determined that the way to have it was to show strength to the Soviets . . . . He sees this summit, this treaty, as the first step in a long and important process."

Reagan's ability to negotiate successfully with the Soviets has in the past been hampered by internal conflicts on arms control that have made it difficult for the administration to speak with a single voice. When Reagan arrived in Geneva on Nov. 16, 1985, for his first meeting with Gorbachev, the pre-summit unity of the U.S. team was disrupted by unauthorized disclosure of a letter from Defense Secretary Caspar W. Weinberger to Reagan warning him of the danger of continued adherence to the SALT II treaty.

The president's national security adviser at the time, Robert C. McFarlane, called the letter a "sabotage" attempt.

In contrast, the run-up to this summit has proceeded smoothly, U.S. officials say. They attribute this in part to greater determination by Reagan to make progress and in part to more harmonious working relations within the administration after the departure of Weinberger and his replacement by former national security adviser Frank C. Carlucci. He in turn was replaced by his former deputy, Lt. Gen. Colin L. Powell, who also works well with Shultz, officials said.

"People are working off the same play book now," a senior official said.

But the internal conflict has been replaced by a struggle between the administration and Republican conservatives who fear the climate of improved U.S.-Soviet relations and are angered by Reagan's attempt to make the INF treaty a test of personal and party loyalty. Reagan last week described treaty opponents as "ignorant," persons who "basically down in their deepest thoughts have accepted that war is inevitable and that there must come to be a war between the superpowers."

Both Shultz and Baker said in television appearances yesterday that the president's remarks were not directed at Senate Minority Leader Robert J. Dole (R-Kan.), who has said the treaty should be examined carefully. But Baker, appearing on NBC's "Meet the Press," said it also was the president's hope that the treaty would be approved without "amendments or reservations" that would require going back to the Soviets for agreement.

Privately, some senior officials say they expect the tension within the Republican Party to be particularly severe between now and the March 8 "Super Tuesday" primaries as GOP candidates slug it out for the 1988 presidential nomination. These officials expect the tension to lessen if the GOP nominee is either Vice President Bush or Dole.

Personifying the change in Washington is Reagan's top arms control adviser, Paul H. Nitze, 80, a leading figure in the 1970s in forming the Committee on the Present Danger, which called for military budget increases and opposed the SALT II treaty. Nitze was asked last week by a reporter if the "present danger" would be over if the summit ended with important movement toward strategic arms reductions.

"I don't think that arms control by itself will change that," Nitze replied, saying that improvements in human rights, bilateral issues and regional conflicts are also important. "Changes of that kind take time. You can't just do them through a piece of paper. What's required is a gradual evolution in the U.S.S.R., on its own steam, moving from a particular point of view to one more attuned to what seems to be happening in the modern world."

The timing of a strategic arms control treaty, even if Reagan and Gorbachev agree on the broad outlines of a compromise, is considered especially critical because of the presidential election and the opposition already expressed by Senate conservatives toward the much more limited INF treaty.

"Opposition to INF among the conservatives is likely to grow in proportion to their perception of the likelihood of a START treaty," said a senior official, referring to a proposed strategic arms reduction treaty. "If it looks like we're going to have a START treaty, INF may become harder to ratify because the opponents will drag out the ratification procedure."

The official said it is considered unlikely that Gorbachev would meet Reagan in a Moscow summit to sign a START treaty unless it were clear that the INF treaty was assured of ratification.

One unusual proposal that has been discussed by Carlucci and received favorably by such INF treaty supporters as Sen. Richard G. Lugar (R-Ind.) is the possibility that Gorbachev and Reagan could essentially approve a treaty that would be presented to the Senate by the next president.

The conventional wisdom is that a START treaty would face difficult hurdles in the Senate if Reagan is succeeded by a Democratic president. But some U.S. officials think that this would be less true if Reagan has put his stamp on the accord even if it were completed after he was out of office.