WHATEVER IT TURNS out to be, year two of the Reagan administration will not be -- cannot be -- a repetition of year one. Year one was one of the most remarkable demonstrations of presidential leadership in modern political history. Year two will test that leadership under far more arduous conditions than President Reagan faced in 1981.

The single biggest difference is that Reagan will find it much harder to control the agenda of government than he did last year. In 1981, he focused the energies of the presidency, the activities of Congress and the attention and support of the public almost entirely on the sweeping budget, tax and defense measures he put forward.

Jeff Fishel, professor of government at American University and author of the forthcoming book, "Presidents and Promises," said Reagan's performance in that regard was "virtually unparalleled" in the modern presidency. "The major difference between Reagan and (Jimmy) Carter and almost all his other predecessors," Fishel said, "is that the Reagan administration simplified its priorities and sacrificed all other concerns to the goal of dramatizing its own program. And that has paid off handsomely for them -- so far."

In planning for 1982, Reagan strategists would like to return to the mixture as before, with economic measures in the forefront and defense and foreign policy as secondary themes. David R. Gergen, the assistant to the president for communications, said last week that the ratios might change somewhat -- "60-40 economics over national security policy, rather than 80-20 or whatever it was in 1981." But he made it clear the two subjects would again virtually monopolize the president's chosen agenda.

But Richard T. Wirthlin, the president's pollster and a principal transition-period designer of the 1981 agenda strategy, conceded in a separate interview that "there will be much more pressure for him (Reagan) to fragment and lengthen his agenda this year."

The reasons why Reagan will have to fight for control of the agenda are many:

The 1981 program was a direct outgrowth of the 1980 campaign, when Reagan promised to attack big government and bolster national defense. But the mandate that led to last year's swift recasting of long-entrenched economic and military policies is now a year older and the paint is peeling.

The economic problems Reagan inherited from the long-dominant Democrats, which provided the rationale for his revisionist budget and tax policies, are now his problems. A recession has compounded what he called last February "the worst economic mess since the Great Depression." The record deficits that loom ahead are his responsibility -- not someone else's. Reagan's close friend, Sen. Paul Laxalt (R-Nev.), said last week that "so much depends on what happens to the economy. If we get the rebound we're looking for, then things can go well. If not, there will be difficult days."

The Democratic Party has begun to recover from its post-election shock and find its voice enough to harass the Republicans at hostile congressional committee hearings -- even if it lacks a program of its own or the strength to impose it on the president.

More important, the remarkable loyalty and cohesion of the GOP is beginning to fray under the pressures of election-year parochialism. Two weeks ago, in what Laxalt called a "storm-cloud" warning, 13 Republican senators from Northeastern and Midwestern states signed a bipartisan letter to budget director David A. Stockman expressing "concern about reported reductions in domestic spending and advising you we cannot support a budget that exacts such a heavy toll" on their region. The signers included not just liberal Republicans but such conservative stalwarts as Sens. Alfonse D'Amato of New York, Dan Quayle and Richard G. Lugar of Indiana and Robert W. Kasten Jr. of Wisconsin.

Congress was beginning to show signs of rediscovering its institutional independence from White House control late in 1981 and, as Wirthlin said, "it is bound to be a more adversarial relationship in an election year when members have to think of their parochial interests." Again, the change is likely to be more significant among Republicans than among Democrats. In mid-December Senate Majority Leader Howard H. Baker Jr. of Tennessee, who led Reagan's fights and won them all, said it was time to think not about more cuts in discretionary domestic programs but about tax increases, defense cuts and trimmer entitlement programs. On the first two points at least, Baker's views contradict the president's. But he has support from the GOP chairmen of the Budget and Finance committees in what could be a classic challenge on taxing and spending decisions.

The challenge is not confined to Congress but includes a reassertion of the typical bureaucratic imperatives of specific departments and agencies. In 1981, there were few endruns around the White House and OMB by agency heads seeking protection for their own turf from sympathetic congressional committees. Reagan demanded Cabinet loyalty and got it. But there has been much more spirited resistance to the 1982 budget decisions from Cabinet members -- including, among others, secretaries Samuel Pierce of HUD, Richard Schweiker of HHS, Malcolm Baldrige of Commerce and James Edwards of Energy -- and those fights are likely to spill over onto Capitol Hill. "You can just see the private agendas beginning to come out again," said one administration insider. What worries him, he added, is that gaps in the White House policy process and signs of tension among Reagan's "Big Three" deputies -- Edwin Meese III, James A. Baker III and Michael K. Deaver -- may weaken the president's ability to maintain discipline for a second year.

Finally, the world, which obliged Ronald Reagan by holding its peace for the first 10 months of his tenure, is showing warning signs that it will demand much more of his attention. The Polish crisis and the disagreements within NATO on how to respond to it, the deteriorating relationship with Israel and the unresolved tensions in the Middle East, the continuing communist-backed civil wars in the Caribbean and the uncertain future of the arms talks all impose their own pressures -- and timetables -- on the president.

None of this means that year two will necessarily be less successful for Reagan than year one. But it does mean that it will take more work -- and, very probably, different skills -- for him to succeed.

Look back upon 1981 with wonder, o ye former presidents, Cabinet officers and White House counselors who were declaring, just a year before, that American government was trapped in an impasse that only constitutional change could cure. Give us a six-year presidency, they said. Give the president authority to dissolve Congress and force a new election if it balked at bringing his program to a vote. Roll back the "reforms" that decentralized power in Congress, that emasculated the political parties and emboldened single- interest groups.

It's possible that Ronald Reagan never read their words. In any case, he proved that a president and his party, using nothing more than the existing tools of politics and persuasion, were all that were needed.

Even discounting the rhetorical puffery in the White House's own year-end review, it was a stunning year. The largest tax cut in American history. A dramatic slowdown in the growth of domestic programs and elimination of many of them. A major effort to reduce the regulatory structure. A redefinition of the relationship between Washington, state capitols and city halls. All this at the same time the commitment of resources to expanding and improving national defense was given a sharp upward tilt.

To understand what made this possible, one must start with two unusual characteristics of the 1980 election.

First, it was much more of a pa. orty election than most in this ticket-splitting era. The Republicans had spent four productive years, after Watergate and the loss of the White House, building their financial and organizational base. For the first time in the television age, a political party bought institutional advertising, designed to enhance its image and depict the opposition as a failure. Reagan's campaign, though focused on the White House, tied itself closely to the Republican Party and its candidates and themes.

For the first time in a generation, the Democrats suffered a defeat that they could not blame simply on the character of their presidential candidate. They lost their Senate majority for the first time in 26 years and -- as subsequent roll calls showed -- they lost de facto control of the House as well.

Second, 1980 was much more of a mandating election than most. The issue agenda was narrow. Economic concerns were uppermost on people's minds. The sense of diminished national power -- going back to Vietnam but symbolized in 1980 by the Iranian hostage ordeal -- was widely felt. There was also a conservative social-issue constituency, but it was smaller and less central to Reagan's victory.

As a candidate, Reagan talked directly to voters' major concerns. He promised to attack inflation and economic stagnation by going to what he said was the heart of the problem -- government taxing and spending policies that drained strength from the private sector. He promised to attack the sense of national impotence on the world scene by talking tough and backing his words with an expanded arsenal of arms.

As president, Reagan used his inner Cabinet officers and top White House staff to pursue those policy goals -- and prevent others from getting in their way. Reagan served as spokesman and chief advocate for the program and, remarkably, congressional Republicans accepted a degree of party discipline that is characteristic of a parliamentary system and almost unprecedented in our recent era of nonparty politics.

James L. Sundquist, Brookings Institution authority on executive-legislative relations, said last week that "the cohesion and unity of the Republican party in Reagan's first year was phenomenal by any standards. The last time Congress followed presidential leadership and enacted a host of important measures was during the Great Society period of Lyndon Johnson. But the Democrats did it then not so much by maintaining unity as by the sheer weight of numbers, with their two- thirds majorities in Congress."

On the key economic measures, there was virtual Republican unanimity. On the three main budget votes, there were zero to three defections in the House, zero to two in the Senate. On the two main tax votes, only one Republican in each chamber broke ranks.

But this magic began to fade during the August recess. Instead of being hailed as heroes and heroines in their home districts, Republicans got complaints about high interest rates crippling housing, auto sales and agriculture. House Minority Leader Bob Michel of Illinois came back from Peoria saying that interest rates, then at 20 percent, have "got to give within 90 days" or the Republicans would be in trouble.

There was relief -- but only because the economy slid into a steep recession. Doubts about Reagan's economics spread from Wall Street to Main Street to Capitol Hill. With the recession deflating revenues and pushing up expenditures, Reagan was forced to abandon his goal of a balanced budget by 1984 -- a dream that had been a powerful incentive to fiscal discipline for many in Congress.

With that constraint removed, interest groups became bolder and the parochial forces which tend to balloon budgets reappeared on Capitol Hill. Instead of praising Congress' cooperatiion, Reagan threatened to veto its "extravagance," and carried out his threat on Nov. 23, using a continuing resolution as his target.

It was in this atmosphere of rising challenge to the principles of Reaganomics that David Stockman's interview appeared in The Atlantic. The Stockman article was damaging because the intellectual credibility of supply- side theory -- the belief that had painfully gained ascendancy within the GOP -- was an important ingredient in the remarkable party cohesion behind the Reagan program. After Stockman's public doubts, it became harder to be a true believer.

As a final piece of the changed picture, Congress showed clear evidence from Labor Day onward of returning to the path of institutional independence -- not to say orneriness -- which had characterized most of the 1970s. That was true not only on domestic issues but on crucial questions of defense and diplomatic policy as well.

The pleas and pressures of politicians and church groups in Nevada and Utah prevailed over Pentagon advice on the basing of the MX-missile system. Lobbying by supporters of Israel came within a handful of votes of blocking the transfer of sophisticated aircraft to Saudi Arabia.

There was increasing static from state and local officials about the impact of Reagan's tax and budget policies on their operations; again, Republicans like Vermont Gov. Richard Snelling and Indianapolis Mayor William Hudnut III were the most vocal critics.

Despite these trends, the administration pulled its forces together at the end of the session and passed a second budget resolution meeting the president's minimum standards. But the handwriting was on the wall: The easy automatic victories were things of the past. As Michel, the architect of Republican unity in the House, said last week, "It's going to be more difficult in the new year. We went through a lot to get $4 billion cut from the budget in December; now, I hear they are asking for another $30 billion. That is going to be tough."

That emphatically does not mean that Ronald Reagan has enjoyed his last triumph. What it does mean is that whatever he achieves from now on, he will have to get the hard way -- by hand-to-hand combat with congressional factions and interest groups, and with less help from the increasingly skeptical Democrats.

As Michel noted in an interview, "it will be difficult to lump everything together in a single package," as Republicans did on the tax, budget and defense bills last year. "We're obviously going to have to address more specific issues in 1982 than we did in 1981. Decontrol of natural gas, for example. Entitlement program cutbacks, for another."

That means Reagan's success will depend on patching together different ad hoc coalitions on each issue. That kind of politics -- with which his predecessors became all too familiar -- will require more than the Great Communicator abilities Reagan displayed this past year. Instead of being able to concentrate on one fight at a time, he will probably be forced to fight on many fronts at once. That will test the range of his political skills and knowledge, and require a greater reach than he had to display this year.

So far as Wirthlin and Gergen, Laxalt and Michel are all concerned, the keystone of Reagan's second year, like his first, will be the budget. All four are of the view that foreign policy will bulk larger on the president's schedule, in part because of the problems now erupting from Poland to El Salvador, in part because preliminary steps have now been taken that allow the contemplation of such events as a Reagan-Brezhnev summit.

But as a matter of both legislative and political strategy, all four think Reagan needs to keep his economic program in the forefront. Wirthlin notes that even though inflation is mentioned as the biggest national problem by only half as many people as last January, the worrisome condition of the economy is still the biggest concern of 60 percent of the voters. Laxalt says simply that "congressional-executive relations all depend on how he casts his budget."

While people outside the White House speculate that Reagan might try to move crime and the social issues to the forefront, there is little inclination among his advisers to recommend that course oeared in Tf action.

Laxalt said he thought crime and the social issues "will get more attention at this end of Pennsylvania Avenue, and I hope the president will get into them a little more." But at this point, Gergen said, budgetary and policy considerations weigh so heavily against an ambitious crime program that it may not even be included in the State of the Union address. Wirthlin's surveys show crime a major issue but not one people expect the federal government to solve.

As for busing, abortion and school prayer, the fear among legislative and political strategists is that they are so divisive within Republican ranks that they must be handled with care. "I don't think the social issues can be ignored," Wirthlin remarked, "but they cannot become the dominant theme."

So both substantive and political considerations point the Reaganites toward an effort to keep economics at the center of the agenda. The good news from Wirthlin's survey is that only one-third of the voters believe that Congress has passed almost all of Reagan's economic program already, and the other two-thirds think more than half the legislative work remains to be done.

That makes it plausible for Reagan to go back to exhorting and pressuring Congress for more budget cuts. But exhorting and pressuring may be much less effective tools in 1982. Both Laxalt and Michel said they think that without some slowing of Reagan's plans for defense spending, domestic budget cuts will be far harder to achieve. "Unless they are given some sort of relief valve," Laxalt said, "those will be tough votes."

So far, Reagan has been unyielding on the defense issue. And that raises the possibility that he could become, in his second year, not the master of Congress but its antagonist. "There is no current appetite" for that, Gergen said, noting that much of Reagan's reputation as a strong, can-do president rests on his ability to work with Congress more effectively than his predecessors.

But political strategist Wirthlin thinks a change is inevitable, if only because "this will be a much, much different Congress in an election year. They will be more parochial and the relationship will be more adversarial. The impression people had in 1981 was that the House of Representatives was really controlled by the Republicans. I don't think that is the impression people will have by the summer of 1982.

"The Democrats will have to flex their muscles against Reagan," he said. "The groups that were burned by the spending cuts will mobilize, and they will put pressure on their congressmen to exempt the programs that are important to them. And if you see that starting to happen, I think you will see Ronald Reagan using his veto much more frequently.

"He may lose some of those fights," Wirthlin said, "but that is not necessarily bad for his image of consistency. And it puts some of the responsibility on Congress -- where it belongs.

"It's funny," he concluded. "Ronald Reagan does not like confrontation. That is not his style. It is not in his nature to put Congress in black hats or say, 'It's us guys against those guys.' But the dynamics of the situation, as I view it, may well see the president pitted against Congress more this year than last year."

Whether that is the best strategy for Reagan in 1982 -- or the only strategy -- is debatable. What is unarguable is that it is as unoriginal and barnacled as the 1981 strategy was novel and bold. Almost every recent president has found himself at odds with Congress on his main objectives, and struggling desperately to keep other issues from crowding his off the agenda.

If Reagan's second year turns out to be of this character, it will mean the return of quirky, patchy, partial, unpredictable policymaking, instead of the bold, dramatic, election-mandated strokes of 1981. It will mean, in short, government as usual.