Four years ago, a confident new Senate Republican leadership moved like a juggernaut to put the Reagan revolution in motion, with dutiful assistance from a bipartisan coalition of conservatives in the Democratic-controlled House.

Now, as President Reagan prepares at the start of his second term to confront another divided Congress, a new set of Senate Republican leaders is prepared to lead the way.

But there is a big difference between 1981 and 1985.

The compulsion, even among shell-shocked Democrats, to follow Reagan's lead in 1981 has given way to second-guessing within both parties that has dramatically reshaped the way the two chambers are going about their business in 1985.

This time, the Senate shock troops are marching resolutely in their own direction, along a brashly independent course that could lead to cooperation or confrontation with the president, or perhaps a blend of both.

House Democrats, plagued as much by internal rifts along generational lines as by the risk of ideological erosion from the right, are just as resolutely sitting out the early skirmishes. They politely acknowledge Reagan's "mandate" to lead and leave it to Republicans to cope with the results.

In the Senate, the prevailing attitude is a bullish, buoyant "What the hell, let's try it," according to Majority Whip Alan K. Simpson (R-Wyo.). Along with other new Senate Republican leaders, he has stepped ahead of Reagan on critical issues, especially those involving budget deficits.

In the House, by contrast, the driving force for the moment is a wait-and-see "strategy of silence," according to one Democratic official.

"The Democrats are being smart, keeping their powder dry and their mouths shut for now," the official added. "It allows the opposition to the president to build first in the Republican Senate, and then the opposition will look bipartisan."

A quick glance at leaders of the two chambers tells a lot of the story.

With a superabundance of energy, ambition, wit and visibility, new Senate Majority Leader Robert J. Dole (R-Kan.), 61, has become the center of attention on Capitol Hill.

Among other things, he lost no time seizing the initiative on deficit reduction, even to the point of putting his troops to work on a budget plan that is to be in his hands before Reagan officially submits his own budget to Congress.

Dole does not challenge the president; he simply smiles and says the Senate wants to "do better."

Rep. Thomas P. (Tip) O'Neill Jr. (D-Mass.), entering his fifth and last term as House speaker at 72, has been far less visible and active during the curtain raisers for the session that begins in earnest after Reagan is sworn in.

O'Neill's public comments about Reagan since the election have been mellow compared with his heated oratory of last fall's campaign. Like one aging patriarch to another, O'Neill made complimentary comments about White House staff changes and administration arms-control efforts, and Reagan reciprocated with a card on O'Neill's birthday and a congratulatory telephone call when O'Neill was reelected speaker.

"O'Neill felt partisan before the elections, but you don't beat up on a guy who's won 49 states," a House leadership aide explained.

Not only the leadership has changed.

Both chambers emerged from the November elections with a more moderate focus as majority parties in each suffered losses, leaving Democrats with a 252-to-182 edge in the House, where one seat is vacant, and Republicans ahead, 53 to 47, in the Senate.

Neither chamber is as conservative as four years ago. Moreover, after pulling apart following the 1982 elections, they have now moved more toward the center and each other. This confluence could spell trouble for Reagan, especially for his more conservative initiatives.

Although Reagan remains in command of the congressional agenda to the extent that there is little serious debate over total reversal of direction, he faces possible trouble on issues ranging from budget priorities to U.S. policy toward Central America.

To win funding for the MX nuclear missile and the program of "covert" aid to anti-government forces in Nicaragua, Reagan must seek votes in both chambers and faces dim prospects on each issue. Votes on them could come within two months.

The main focus of the first session of the 99th Congress is expected to be budget deficits, and Senate Republicans are moving quickly to fill a vacuum.

The White House has conceded that it cannot produce spending cuts to halve deficits to less than $100 billion within three years, and Democrats are content to let Reagan founder for the moment. But Senate Republicans are trying to write an alternative budget, including a spending freeze coupled with selected program curtailments borrowed from Reagan's anticipated budget, with a deficit of $100 billion or less as its goal.

Difficult, perhaps insurmountable, problems loom on specifics. SS enate Republicans are talking of dropping inflation S adjustments for Social Security for next year; House members of both parties are balking. Although many Senate Republicans favor an across-the-board spending freeze, they are far from unanimous about freezing military spending. Some of the selected program cuts, as drastic as any proposed by Reagan over the last four years, are also expected to cause major controversy.

If failure to produce adequate spending cuts leads to pressure for a tax increase, the White House can be expected to act against it.

Moreover, issues left by the 98th Congress, ranging from civil rights to environmental policy, touch sensitive points on the Reagan agenda and are bound to arouse sensitive constituencies nationwide.

Finally, in both chambers, Reagan must deal with new committee chairmen who, in many cases, may be tougher to handle than their predecessors.

The House Permanent Select Committee on Intelligence, which oversees Nicaragua funding issues, will be headed by Rep. Lee H. Hamilton (D-Ind.), a softspoken, well-respected lawmaker who last January helped lead the effort to force the administration to withdraw U.S. Marines from Lebanon. Like his predecessor as head of the panel, Rep. Edward P. Boland (D-Mass.), Hamilton has strongly opposed continued funding for the Nicaraguan rebels.

The House budget battle will be led by a new Budget Committee chairman, Rep. William H. Gray III (D-Pa.), who, in his months-long effort to win the chairmanship, distinguished himself as a preeminent political infighter.

Gray, who represents an inner-city district in Philadelphia, is considered more liberal than his predecessor, Rep. James R. Jones (D-Okla.), and certain to be as aggressive as Jones in fashioning a Democratic budget alternative.

In addition, Gray, who is black, is viewed as the perfect Democratic spokesman to raise the "fairness" issue against the administration. He is one of five blacks now heading standing House committees. AA nother new face likely to give the administration A heartburn is Rep. Les Aspin (D-Wis.), an acknowledged expert on defense issues and longtime Pentagon critic. He was elected chairman of the Armed Services Committee after Democratic colleagues voted out incumbent Rep. Melvin Price (D-Ill.), 80, who was viewed as too frail and too supportive of the Pentagon.

Aspin, who helped the administration win support for the MX two years ago, has indicated to some colleagues that he may be rethinking his position. An Aspin shift would be a severe blow for the administration, which also faces mounting Senate opposition to the missile.

In general, Aspin is expected to conduct more aggressive and skeptical oversight of the Pentagon.

Aspin's election over six more senior committee members was a sign of generational upheaval in the House. Younger Democrats elected since 1975 now constitute about 70 percent of the House. Less wedded to traditions of seniority, they have pressed for greater influence and a new party image tied less to the New Deal and focused more on challenging the GOP for new ideas.

This issue is expected to play a large role in the 1986 House leadership race when a successor to O'Neill is selected. Aspin's selection, for instance, was viewed by some as a shot across the bow of Majority Leader James C. Wright Jr. (D-Tex.), O'Neill's hand-picked successor. No challenger has emerged, however.

Most House Republicans are expected to remain loyal supporters of the president. But here, too, turbulence about the party's direction has occurred, mostly between a group of younger members, such as Reps. Jack Kemp (R-N.Y.) and Newt Gingrich (R-Ga.), adamantly antitax, and more moderate Senate Republicans, such as Dole, who have said they would seek to raise taxes if necesssary to reduce deficits.

In addition, last fall's elections increased the ranks of "Gypsy Moth" Republicans from the Northeast and Midwest. They often opposed Reagan budget cuts during his first term, and have decided to make a greater push this year to counter the influence of more conservative Republicans. The result could be more difficulty for the White House in gaining unified GOP support for its programs. II n the Senate, Reagan not only faces a more aggressively I independent-minded leadership than he did under retired majority leader Howard H. Baker Jr. (R-Tenn.) but also will find some mavericks as new chairmen of major committees.

Instead of Pentagon loyalist Sen. John G. Tower (R-Tex.) at the Armed Services Committee, there is Sen. Barry Goldwater (R-Ariz.), who has suggested, among other things, that the administration drop its advocacy of the MX, which it has shown no signs of doing.

Replacing Dole as chairman of the Finance Committee is Sen. Bob Packwood (R-Ore.), whose independent streak has rankled the administration, although he has generally agreed with its tax policies.

Replacing Goldwater on the Select Committee on Intelligence is Sen. David F. Durenberger (R-Minn.), who has criticized continued aid to the Nicaraguan rebels, which Goldwater supported.

The administration is expected to receive more help from Sen. Richard G. Lugar (R-Ind.) as chairman of the Foreign Relations Committee than it did from his predecessor, Charles H. Percy (R-Ill.).

With the Republicans' Senate margin narrowed by four seats, the Democrats need to pick up only that many votes to prevail on specific issues, as long as they can stick together, which they have had trouble doing in the past.

But, like the House Democrats, they are lying low so far, and the half-dozen moderate-to-liberal Republicans who have bolted from the GOP position in the past, especially on social-welfare issues, appear to be mollified if not delighted by the initiatives of Dole & Co.

Although many Democrats spoke enviously of the new image given Republicans by Dole, they passed up a chance to replace their leader, Sen. Robert C. Byrd (D-W.Va.), despite private grumbling about his nuts-and-bolts style of leadership.

Unwilling to unite behind the only challenger, Sen. Lawton Chiles (D-Fla.), more conservative than most Senate Democrats, they appeared willing to hang on at least another two years, when they are counting on next year's electoral arithmetic to put them back in the majority.

With Republicans having to defend 22 of 34 Senate seats at stake in 1986, Democrats appear to have a good chance of adding at least four seats, enough to reclaim control.

Republicans' downbeat mood about 1986 has perked up considerably since the new leadership took over, even though none of its initiatives have passed even a first test.

"There's this crazy, naive feeling that nothing's impossible now, even keeping hold of the Senate," one Republican said, "and, if you really believe this sort of thing, you can sometimes make it happen."