It was the Republican takeover of the Senate -- not the overwhelming victory of Ronald Reagan -- that truly destroyed the self-confidence of liberal Democrats.

Democrats had come to believe that the Senate was theirs by divine right.Republican presidents had come and gone, but the Senate had remained Democratic for 46 of the last 50 years.

No individual better symbolized the worst nightmares of Democratic liberals than Sen. Strom Thurmond (R-S.C.), the onetime foe of integration, who took the chairmanship of the Senate Judiciary Committee away from Sen. Edward Kennedy (D-Mass.).

Two days after the election, which gave the Republicans a 53-to-47 Senate majority, Thurmond held a press conference and stated his support for a federal death penalty. That remark become front-page news because it clearly seemed to illustrate the shape of the newly emerged Republican majority.

There was just one problem with this interpretation -- the liberal Senate Judiciary Committee, by a vote of 7 to 4, had already sent the death penalty bill to the Senate floor as part of an elaborate compromise between Kennedy and Thurmond over revision of the U.S. criminal code.

Thurmond's bill died in the waning days of the 96th Congress, but the incident illustrates why it is dangerous to jump to hasty conclusions about changes that will be wrought by a Republican Senate.

Obviously, the Reagan administration will have an easier time on Capitol Hill with the Senate in Republican hands. Despite the extreme pronouncements of a few new committee chairmen, the Senate under the Republicans will not be much different from the body that was controlled by the Democrats, 59-to-41.

Some familiar faces, such as George McGovern (D-S.D.) and Frank Church (D-Idaho), will be gone, replaced by a noisy band of conservatives, some of whom were elected with the help of the Moral Majority. But much of this thunder on the right will be "sound and fury signifying nothing."

It is easy to misperceive the Republican Senate by contrasting it to the liberal Senate of the 1960s dominated by charismatic figures such as Hubert Humphrey. But that Senate is long gone, destroyed by a conservative tide that began with the election of 1976.

From the opening days of the last Congress, when the Democratic Senate voted to lift the sanctions against Rhodesia, to the closing lame-duck session in December, when the open-housing bill was filibustered to death, Democratic control of the Senate did not provide a fertile ground for liberal legislation.

There will not be much liberal legislation for the Senate to kill in 1981, although the Republicans will not try to turn the clock back, either. The Bill of Rights will not be repealed, war will not be declared on Denmark and the federal government will continue to mail out Social Security checks.

Instead, there will be more oversight of social welfare programs, tighter eligibility formulas, holy war will be declared against waste and corruption, taxes will be cut and defense spending increased.

But the Senate will remain an inherently moderate body operating by consensus, despite the change in party leadership.

Howard H. Baker Jr. (R-Tenn.), the new Senate majority leader, echoed this refrain when he said recently, "There is an old saying among lawyers about the Supreme Court that the Court changes men more than men change the Court. The same thing may be true here in the Senate in terms of majority responsibility and committee chairmanships."

In part, Baker was referring to the new moderate image of Sen. Orrin Hatch (R-Utah), new chairman of the labor committee. Hatch, elected to the Senate in 1976 as part of the "Sagebrush Rebellion," became public enemy number one to the labor movement when he led the 1978 Senate fight that killed labor law reform.

Within two weeks of the election, Hatch had a cordial lunch with AFL-CIO President Lane Kirkland and stated that he no longer favored repeal of the Occupational Safety and Health Act (OSHA), one of the landmark liberal legislative victories of the last decade.

"When I first came to the Senate," Hatch said in a recent interview, "I thought OSHA should be abolished." Hatch still calls the administration of OSHA "bureaucratic and oppressive," but says experience has taught him that there are indeed "unsafe work places" in the modern United States. Now, Hatch said, "I think there is a need for OSHA."

Hatch may be an extreme example of the moderating influence of power, and his conversion may be more cosmetic than substantive, but he is not alone in the new Republican Senate.

Another Western conservative, Jake Garn (R-Utah), who took over for William Proxmire (D-Wis.), as chairman of the Senate banking committee, sounded a similar note.

"I am a fiscal conservative," said Garn, "but I am also a pragmatic legislator." Garn believes that the banking committee, which also has responsibility for housing programs, should focus on oversight of existing legislation. "I think we have been too active in passing new legislation," he said.

Garn bristled at the suggestion that conservatives who have supported senators such as himself and Hatch might be impatient with their go-slow attitude toward change.

"I think conservatives make a very big mistake when they say if I can't have the whole pie, I don't want any of it," he said. "I don't think that's the responsible way. Rather than stating, if I don't get three-fourths, I'll take nothing, I'd rather get a fourth. Those conservatives who stand on one point and demand the whole hog are not going to get anything."

On election night, a prominent Washington liberal, who had spent much of the 1970s battling the filibuster and congressional seniority system, was suddenly singing the praises of these venerable Senate institutions.

His praise for the filibuster was understandable, since it is now a weapon that Democrats can use to block conservative legislation. As Sen. Alan Cranston (D-Calif.), the minority whip, said, "The Republicans gave us fits with 41 votes in the last session. Now we can give them double fits with 47 votes."

True, Senate Democrats are much more reluctant to use the filibuster as a weapon than are their Republican counterparts.

Sen. Carl Levin (D-Mich.), one of the brightest lights among the few liberals elected to the Senate in recent years, said, "I don't believe in the filibuster. Psychologically, I'm not the right person to do it with gusto."

The seniority system, rather than the filibuster, will do the most to maintain a moderate, centrist orientation for the new Senate.

While new Republicans elected to the Senate in 1978 and 1980 are disproportionately conservative, many of the committee chairmanships in the new Senate will go to Republicans almost as liberal as their Democratic predecessors.

None is more powerful, nor more apt to be at odds with the Reagan administration, than Sen. Mark Hatfield (R-Ore.). One of a handful of senators militantly opposed to higher defense spending, Hatfield is also the new chairman of the appropriations committee.

As Democrat Cranston put it, "It must be awfully galling to the Republicans to finally get control of the Senate and have Hatfield be the one to get appropriations."

Hatfield concedes that he is not going to win many battles on defense spending, but he is in position to derail any attempts by the Reagan administration to make drastic cuts social welfare programs.

"My voice will be raised more frequently than not," Hatfield said in a recent interview, "as one who believes that the social programs cannot bear the full brunt of so-called budget restrictions."

Hatfield indicated that his approach would be different than that of Warren Magnuson (D-Wash.), his traditionally liberal predecessor who was defeated for reelection. Whereas Magnuson would fight to maintain existing programs in their present form, Hatfield would be willing to let others reshape the programs if he could, in exchange, maintain existing funding levels.

Other moderate to liberal Republicans also were rewarded with committee chairmanships because of the seniority system.

Sen. Robert Stafford (R-Vt.), new chairman of the environment committee, made it clear in an interview that he would be willing to battle the Reagan administration over any effort to gut the clean air act.

The internationalist tradition on the much-weakened foreign relations committee will be maintained by Sen. Charles Percy (R-Ill.), the new chairman. Sen. Robert Packwood (R-Ore.) and Charles McC. Mathias (R-Md.) are other progressive Republicans with positions of influence in the new Senate.

No one benefited more personally from the Republican takeover of the Senate than Robert Dole (R-Kan.), the new chairman of the finance committee. Dole, whose abortive presidential campaign became a laughingstock when he received less than 1,000 votes in the New Hampshire primary, emerged like a phoenix after the election as chairman of the most powerful committee in the Senate.

Despite Dole's conservative reputation, earned in part during his partisan efforts as Gerald Ford's running mate in 1976, he is far closer philosphically to Sen. Russell Long (D-La.), now the ranking minority member, than to some of the more zealous devotees of supply-side economics in the Reagan White House.

Asked what would change with him as chairman instead of Long, Dole gave an answer that helps explain why some militant conservatives are apt to be disappointed in a Republican Senate.

"I don't know," Dole began. "It's going to take a while. I don't see any big difference initially. There may be some difference in personalities, but there will be a close working relationship with Sen. Long. If you want any big confrontation or philosphical difference, there just isn't going to be any."

In fact, Dole emphasized that, because Long is not longer chairman, that does not mean that he will cease to be a Senate power.

"Right after the election," Dole said, "I told Gov. Reagan that if he had a half an hour to see Bob Dole, he should just call me and say hello and then spend the rest of the time with Russell Long. I think Long's the key to the finance committee. I think he's still key to the whole Senate."

It is often deceptive to look at the Senate in strict party-line terms. Often it is more useful to view the Senate as a group of contentious committee fiefdoms, each with their own bipartisan ideological majorities.

Committee chairmen tend to recruit like-minded freshmen members. Thus, Slade Gorton (R-Wash.), one of the few moderates elected last fall, joined the commerce committee at the urging of chairmam Packwood. And Thurmond made sure that newly elected John East (R-N.C.), a protege of conservative Jesse Helms (R-N.C.), got a seat on judiciary.

This likemindedness helps explain why Helms, the new agriculture chairman and a man of strong views, said, "I think you'll find that the Ag committee will continue to be what it has always been, a bipartisan committee. There is nothing bipartisan about the problems of the American farmer."

In similar fashion, the armed services committee, under John Tower (R-Tex.), will continue to give any president whatever military spending he wants, and more. There will be one important change on armed services, however, that is not readily obvious, given the overwhelmingly hawkish tenor of the committee.

Had the Democrats retained control of the Senate, this would have been the year for a long, detailed and acrimonious debate on reinstitution of the draft. The stimulus would not have come from the White House, but would have been the pet project of Sen. Sam Nunn (D-Ga.), who would have kept the chairmanship of the manpower sub-committee.

Tower does not share Nunn's belief in the inevitability of the return to the draft. The new chairman contends, "I don't think we've done everything we can to make the volunteer Army work." So much for the great draft debate of 1981.

Understanding change on Capitol Hill is a difficult assignment since congressional procedures often are similar to a magician's sleight-of-hand tricks. It is easy to be mesmerized by the grandstanding right hand while the almost invisible left hand is pulling legislation out of a hat.

That is why many of the headlines coming out of the Republican Senate in 1981 will have little to do with substantive legislation.

Strom Thurmond will increase the apoplexy level of embattled liberals by attempting to kill the Voting Rights Act, and Orrin Hatch, new chairman of the constitutional amendments subcommittee, will attract attention with his proposal to outlaw affirmative action programs.

But when the dust settles, some form of extension of the Voting Rights Act will be approved, and militant conservatives will be concocting conspiracy theories to explain why their pet constititutional amendments can never get through Congress.

"The difference between the loyal opposition and the majority is very real," Howard Baker said. "There is less room for individual initiative when you have the responsibility for moving a legislative package. When you're in the opposition, you can find one piece of high ground and snipe away very effectively. But when you are a chairman and in the majority, there's a different set of imperatives."

And, for a Republican Senate, these imperatives mean more continuity than far-reaching change on Capitol Hill.