In January 1977, when Sen. Robert C. Byrd (D-W.VA.) assumed the Senate's majority leadership, his liberal Democratic colleagues were not happy. Hubert H. Humphrey had been their candidate for the job, and Byrd's election left them disappointed and a little fearful.

Two weeks ago, as the Senate stumbled to the conclusion of a long and exhausting session, many of the same liberals went out of their way to praise Byrd's performance as majority leader. In an interview last week, Byrd himself recalled their words: Sen. Edward M. Kennedy (D-Mass.) had offered "high praise"; Sen. Paul Sarbanes (D-Md.) had called him specially from Baltimore to say how pleased he'd been; Sen. John Glenn (D-Ohio) had called too, as had Howard Metzenbaum (D-Ohio) and others.

Robert Byrd is a fiercely proud man, and he could not resist reciting that list, though he knew - and said - that those other senators "should speak for themselves." In fact, many of the liberals would happily speak out in praise of Byrd.

Kennedy, for example, whom Byrd defeated in a 1971 vote for the post of majority whip, authorized an aide to say that his original doubts about Byrd as majority leader "are no longer justified."

"Byrd has earned high marks for his leadership," Kennedy's statement said. "He was particularly effective not only in helping Senate liberals schedule certain key bills for debate, but also in rounding up critical votes to pass controversial measures like the Panama Canal treaties and the constitutional amendment providing for D.C. representation in Congress."

Kennedy's effusiveness may be partly explained by his impending ascendancy to the chairmanship of the Judiciary Committee, where he will be a Senator elder, dependent on Byrd's help and cooperation. But other liberals share his general approval of Byrd's tenure as leader, and few Democrats are still critical of him.

This culminates a radical transformation in Byrd's image. Ten years ago the West Virginia - whose only hobby is playing a country-style fiddle - was probably best known for his persistent criticism of welfare programs and welfare recipients as chairman of the Senate's appropriations subcommittee for the District of Columbia. He had no political admirers to speak of among Senate liberals then.

Today Byrd can boast - and does - that he steered the Senate through the 95th Congress without alienating any important group in his party, without succumbing to many attempted filibusters, and without leaving any serious possibility that he will be challenged for the majority leadership when the new Congress assembles in January.

"I don't know anyone [among Democratic senators] who wants to get executed," Byrd said when asked about the possibility of a challenge. The remark seemed to be meant in jest, though it was delivered without a smile. "There's more to that remark than facetiousness," he added. "It's a killer of a job."

As Byrd performs it, that is an accurate description. Sen. Mike Mansfield (D-Mont.), Byrd's predecessor, led in a relaxed manner, leaving much of the day-to-day administrative work to his majority whip-Byrd. As majority leader Byrd has kept all that in his own hands, and taken on the political obligations of leadership as well. Byrd regularly devotes 16 or 18 hours a day to the job, maintaining a pace that would deter all but the most ambitious potential challenger.

"It takes a very strange combination of qualities" to be a good majority leader, one Senate aide observed. "You have to be someone who has no interests other than the Senate - the kind of person you wouldn't want to have dinner with." This is a view of Byrd shared by many in the Senate who think of themselves as more sophisticated but also less dogged than Byrd.

It is no accident that Byrd has disarmed his potential rivals - he clearly set out to do so from the beginning. People who have watched him closely for years say he is much more concerned with winning - with his status and authority - than with the substance of any issue, so he can bend his substantive positions to preserve or enhance his strength as leader.

Thus when he saw that liberal Democrats were intensely determined to pass the D.C. voting rights constitutional amendment, he helped them do it. His first inclination was not to let the extension of time to ratify the Equal Rights Amendment come to the floor, knowing that it would produce a bitter fight. But once again enough clleagues - this time aided by a number of senators' wives - convinced him that the issue was important to them, and he took on the fight, which ERA supporters then won.

Byrd's concern about his leadership authority also helps explain the serious strain that developed in his relations with the White House in his closing days of the 95th Congress. According to well-placed sources, Byrd was furious with President Carter for voting the public works bill, not least because he considered the water projects that so angered Carter a traditional congressional prerogative.

According to a Senate aide who was close to Byrd at the time, the majority leader "was just totally distraught" when the House failed to override Carter's public works veto.

Byrd had the votes in hand to override the veto in the Senate, according to this aide, and was just waiting for the chance "to show how powerful he really was."

In fact this would have been Byrd's first serious challenge to Carter. Most of the time he has been a crucial supporter of the White House, most notably on the Panama Canal treaties and other foreign policy battles, but on demestic issues, too.

Administration officials say Byrd's support was crucial to the compromise natural gas pricing bill that Carter eventually designated the key element of his energy program. Earlier this year, at Carter's request, Byrd made one of his rare trips to the major NATO capitals in Europe (he reportedly does not like to fly). European leaders repeatedly told the majority leader how important it was for the United States to take action on energy, and he gave a commitment to several of them to do everything he could. He made good on that commitment in helping deliver a majority for the controversial gas proposal.

Gas was one of many issues to provoke attempts at filibusters during this Congress, and the stalling tactics adopted by some senators annoyed Byrd, sometimes angered him. In the closing days of the session he threatened repeatedly to seek rules changes next year to make delaying tactics more difficult, and in the interview last week he insisted that he is serious about this.

Byrd has already written a letter to Sen. Howard H. Baker Jr. (R-Tenn.), the minority leader, proposing changes in the Rule 22 (the filibuster rule), among others. Baker said when queried that the Republicans would probably not be inclined to support any changes.

But Byrd indicated he will make an attempt to alter Rule 22 so that three-fifths of those present, instead of a flat 60 of the 100 senators, can vote to cut off a debate. He also wants to allow a majority of the Senate to vote to bring up a new piece of legislation at any time. (At present, a "motion to proceed" on a new bill can also be filibustered.)

But the area that may concern Byrd the most is the "post-cloture filibuster," a tactic to delay the Senate even after the necessary votes have been found to invoke cloture.

Senate aides say Byrd was anxious to bring the controversial labor law revision bill to a vote this year to test some parliamentary maneuvers he dreamed up for dealing with post-cloture delaya. But that was one case when the pre-cloture filibuster succeeded-Byrd could not find 60 votes to cut off debate.

The majority leader may seek rules changes in the new session in this area.

"It is unfair to compare Byrd to Lyndon Johnson," the last powerful Senate majority leader, one Senate aide observed. "The Senate rules have really become nonfunctional" since LBJ's time, this aide said. Byrd obviously agrees, and is looking for ways to change the rules so that he, too, can become known as a master of the Senate of Johnsonian dimensions.

"Don't kid yourself about Bob Byrd," one longtime student of the majority leader said recently. "He's running for president too, just like most of the rest of them." Perhaps so. But whatever the ultimate objective, Byrd is clearly determined to make his mark now as the doinant figure in the Senate.