President Reagan will emerge Monday from the sunny seclusion of his mountaintop ranch to face three months of political struggle and foreign policy decision-making that could determine the fate of his second term.

Both White House spokesman Larry Speakes and longtime political adviser Stuart K. Spencer used the word "critical" last week to describe battles facing the administration between Labor Day and Thanksgiving. They also predicted that Reagan will again confound his critics and demonstrate his leadership despite the odds facing a president in the first year of a second term.

Spencer argues that the president has "developed his own personal relationship with the American people" that will enable him to win legislative victories and score at least a public relations triumph in his November summit meeting with Soviet leader Mikhail Gorbachev.

Behind the facade of official optimism, however, some Reagan advisers express concern that the White House, under chief of staff Donald T. Regan, might not be up to the political task of dealing with the complex legislative agenda that Reagan faces this month.

Reagan launches his vaunted "fall offensive" on behalf of his tax-overhaul measure Monday with a speech in Independence, Mo., and plans to continue it throughout the month with speeches in different regions.

But while the White House emphasizes the tax issue and Reagan prepares for the summit, he is likely to be surrounded by political conflict. For example, he has promised to veto appropriations bills that displease him and appears ready to veto legislation imposing sanctions on South Africa.

The administration also must try to find a compromise on a politically sensitive farm bill, increase the federal debt ceiling above the $2 trillion mark and attempt to fend off protectionist trade bills.

"Historically, Reagan has done very well when he has been able to focus his energies on a single issue," one White House official said. "This enables him to dramatize his cause and build public opinion for what he wants to do. He has been less successful when he has to juggle lots of balls in the air at the same time."

This point was illustrated earlier this year when the controversy about Reagan's visit to a West German military cemetery at Bitburg coincided with key congressional votes. The administration narrowly lost an important test on aid to rebels in Nicaragua, later reversed, and some White House officials attributed it to the Bitburg diversion.

Now, Reagan faces a crowded and conflicting agenda. His appeal for tax revision depends on bipartisan support in the Democratic-controlled House, which calls for a conciliatory approach. But his determination to hold the line on government spending by vetoing bills that depart from his budget guidelines could lead to confrontations that some officials say could jeopardize tax revision.

Concern about this emerging contradiction has been voiced within administration councils by Treasury Secretary James A. Baker III, the former White House chief of staff who swapped jobs with Regan earlier this year. But there is no sign that the views of Baker, a frequent engineer of political compromises during Reagan's first term, are being heeded in the White House.

"Don Regan doesn't seek Jim Baker's advice, and Jim Baker doesn't offer it," a senior administration official said last week.

Members of the Regan team, whose competence has been questioned by veteran Republican members of Congress and politicians, dispute the view that the president's agenda is in trouble and that Regan is over his head.

"Expectations are so low about what we'll be able to accomplish that it may be a help to us," said a White House official who says he thinks that Reagan can steer a course between confrontation and bipartisanship by "judicious" use of veto power.

History paints a more pessimistic picture. Even such popular presidents as Franklin D. Roosevelt and Dwight D. Eisenhower found the first year of their second terms especially difficult. Reagan's position is clouded by questions stemming from his surgery for colon cancer July 13.

Also, at home and abroad, the administration faces opportunities unlikely to be repeated.

National security affairs adviser Robert C. McFarlane has pointed out that the summit meeting in Geneva Nov. 19-20 comes as the Soviet Union and United States are about to deploy new strategic nuclear weapons systems. If they reach no agreement on a new arms-limitation treaty, it may be years before a similar opportunity presents itself.

The administration also views the summit as a chance to practice "public diplomacy" advertising the president's proclaimed desire to find areas of agreement with the Soviets. Robert J. Korengold, the U.S. embassy's public affairs officer in London, will be brought in to head this effort, officials said.

Domestically, GOP political strategists now give their party less than a 50-50 chance of holding on to the Senate in 1986, when 22 Republican incumbents and only 12 Democratic incumbents face reelection. Since legislative accomplishment is difficult for any president in an election year, this may be Reagan's last chance to exploit GOP control of the Senate.

White House officials say Reagan is almost fully recovered physically and ready to face a difficult political challenge with his customary optimism.

The 74-year-old president seemed in good health and high spirits as he posed for pictures Wednesday at the annual White House press party here. He has lost a few pounds since his operation, and his tan has faded because he has followed doctors' orders to wear a cap outside since a cancerous pimple was removed from his nose a month ago.

A doctor was in attendance at the ranch throughout Reagan's 23-day California vacation, and the president is to be X-rayed soon after returning to Washington to make certain there has been no recurrence of the malignancy in his colon, Speakes said. Reagan has spent 201 days at the ranch since his election as president.

Reagan was described as eager to resume campaign-style speeches for his tax plan.

"This fall is an important and critical time for the Reagan administration," Speakes said in an interview last week. "We will spare no effort to take our case to the people on tax reform, and we expect people to respond to the president."

After speaking in Independence, Reagan plans a trip to Raleigh, N.C., Thursday to speak at North Carolina State University. Other tax speeches are planned this month in Oklahoma, Texas and a site in the Northeast.

While Reagan is campaigning throughout the country for tax revision, Baker and trouble-shooting deputy Richard G. Darman will be attempting to reach an accommodation with House Ways and Means Committee Chairman Dan Rostenkowski (D-Ill.).

Administration officials are more bullish about winning House approval of the tax bill than any major item on Reagan's agenda, in part because they think that Democrats would like to cast themselves as champions of tax revision. Here is the administration outlook on other key issues:

*TRADE -- Reagan is determined to resist the more than 300 protectionist bills introduced in Congress, as shown by his decision last week rejecting shoe-import quotas. The administration worries that various industries will band together to push an import surcharge on several types of goods. That would be harder to resist than several separate bills.

To defuse the protectionist sentiment, the White House plans to push ahead with administrative actions directed at unfair trading practices. A probable first target is Taiwanese infringement of U.S. copyrights and patents.

*DEBT CEILING -- The need to increase the debt ceiling above the milestone mark of $2 trillion, double what it was when Reagan came to office, is likely to give liberals and conservatives the opportunity to add ideological amendments.

"The debt-ceiling bill could come to us with all sorts of bells and whistles on it, and Reagan might have to veto it to get a clean bill," a senior official said.

*BUDGET -- Despite Reagan's fondness for talking about his veto pencil, he has vetoed only four bills as president. Administration officials are divided as to whether he should be highly selective in his vetos or reject any spending bill that violates his guidelines.

*FARM BILL -- The administration is counting on Senate Majority Leader Robert J. Dole (R-Kan.) to devise a compromise on a bill regarded as a budget-buster. "Paradoxically, this is the easiest veto to sustain, but it could also be the most damaging politically in terms of Republicans who are up in '86," one White House official said.

*SOUTH AFRICA -- Reagan will veto any sanctions bill, but officials said he will also take administrative actions, such as banning computer sales to South African agencies that discriminate, in the hope of heading off a veto override.

Again, administration officials differ on the consequences of such a veto. Spencer said the South African situation could get "messy" for the administration. And others said that, once a Reagan veto is overridden, it will be easier to override others. But other officials think that Republicans, once showing their independence on South Africa, will be more inclined to loyalty on economic issues.

The biggest unanswered question within the administration is whether chief of staff Regan's much-criticized corporate style of leadership can adapt to changing political realities in Congress.

The president is blamed by leading GOP senators for scuttling a Senate-engineered budget bill that would have reduced the deficit, considered by many the greatest threat to his second-term success.

Some Reagan advisers complain that Regan's hierarchal style deprives the president of the wide range of advice he received during the first term when Baker, deputy chief of staff Michael K. Deaver, counselor Edwin Meese III and national security affairs adviser William P. Clark often gave Reagan a wide range of options. "Reagan's very good at picking from options but not good at initiating them," one official said. "Regan reinforces the president rather than complementing him and seeing that he has contrary views."

Regan's usual response is that he is simply letting "Reagan be Reagan" and carry out the agenda of his presidency.

"I take issue with the common wisdom that the second term should be conciliatory," Regan said in an interview with Business Week magazine. "If Reagan were to dilute his programs now, his position in history would suffer."

However, some officials say Regan, without publicly acknowledging error, has recognized that his political inexperience has been costly. They cite his hiring of Dennis Thomas, considered an effective lobbyist when he worked for Regan in Treasury, as a White House aide and call it an indirect acknowledgement of weakness in dealing with Congress. "He's made some mistakes, and we've got to hope he's learned from them," Spencer said.

Regan's problems are compounded by the prospective departures of veteran political aides Edward J. Rollins and Max L. Friedersdorf from a White House already keenly feeling the loss of Baker, Darman and Office of Management and Budget Director David A. Stockman, who was also effective on Capitol Hill.

The Regan team is inexperienced and deferential to him. Among senior officials, only Speakes and White House communications director Patrick J. Buchanan are experienced White House hands.

Ultimately, however, the key is in the hands of the president and First Lady Nancy Reagan, who has seen to it that high officials who fail to perform find new positions.

Reagan, counted out by many early in his second term in California and then again in the middle of the 1982-83 recession, has made many comebacks and now has exceedingly high popular approval.

"We think the president can do it again because he's always done it," said one veteran Reaganite last week. "But the economy isn't going to ride high forever, and even Reagan can run out of time."