A new generation of leaders took control of the Senate yesterday. Robert C. Byrd of West Virginia, an unknown nationally until a half dozen years ago, was chosen unanimously as Senate Democratic majority leader and Howard H. Baker Jr. of Tenneessee won the Republican leader post in a surprising 19-to-18 upset over Robert P. Griffin of Michigan.

Byrd, 58, assistant Democratic leader for the past six years under retiring Majority Leader Mike Mansfield of Montana, was elected without opposition after Hubert H. Humphrey of Minnesota, his only opponent, withdrew because he knew he would lose by a large margin. With a Democratic President, most Democratic senators believed they needed a skilled manager of Senate business, which Byrd has proven to be, rather than a national party spokesman such as Humphrey.

Humphrey, 66, a former Vice President and onetime presidential nominee of his party now recuperating from cancer surgery, said he called Byrd at 8 a.m. to tell him he was withdrawing. "I felt that I did not have sufficient votes and it was in the interest of the Democratic majority to come out of this caucus united," he told reporters.

Byrd, in a gesture to ease the pain of defeat for Humphrey, said the Minnesotan didn't need any titles to be a national leader. "He is a national leader, he has been a national leader, and he will always be a national leader," Byrd said. Nevertheless, Byrd created a three-man study unit to recommend some "special mark of recognition for Humphrey."

Democrats also chose Sen. Alan Cranston of California, 62, who was unopposed, as assistant leader, there-by balancing Byrd's middle-of-the-road political stance with a party. They elected Daniel K. Inouye of Hawaii as secretary of the Democratic $99[WORD ILLEGIBLE] the conference of the 62 Democratic senators.

An anticipated attempt to oust Harry F. Byrd Jr. (Ind,-Va.) from the Democratic Caucus because he runs as an independent though he continues to sit with the Democrats in the Senate didn't materialize.

Republicans, after giving Baker his one-vote, nonideological victory over the heavily favored Griffin in the fight to succeed the retiring Hugh Scott of Pennsylvania as GOP leader, elected Ted Stevens of Alaska without opposition as assistant Republican leader. They also ousted Robert T. Stafford of Vermont, a member of the party's liberal wing, as secretary of the GOP conference in favor of conservative Clifford P. Hansen of Wyoming on a 20-to-17 vote.

The victory for the 51-year-old Baker on his third try (he lost twice to Scott) came on the birthday of his late father-in-law. Everett McKinley Dirksen of Illinois, Senate Republican leader for many years.

Griffin, assistant GOP leader for the past seven years, went into the GOP conference believing he had the votes of 20 to 21 of the 27 Republican senators present. (One GOP senator, Charles H. Percy of Illinois, was absent, vacationing in Europe.

However, a band of GOP senators, convinced the party needs a new, articulate Senate spokesman with a new face and a new image, had been working for Baker for some time, led by Robert W. Packwood (Ore.), Peter V. Domenici (N.M.), Dewey F. Bartlett (Okla.), Charles MacMathias (Md.) and others.

Mathias placed Baker's name in nomination. Mathias told the GOP conference that the party needed a symbolic change from its old leadership and should send a message of change to the country by electing a new, clean-looking face, a nan considered articulate and a good party spokesman.

Domenici, seconding the nomination, told the causus, "We are a small band with a rare opportunity." Milton R. Young (R-N.D..) nominated Griffin and Mark Hatfield (R-Ore.) seconded.

One factor that may have worked against Griffin, aside from the feeling that Baker would make a better, more articulate spokesman for a party whose most visible leaders will now be those in Congress, is the distillusionment felt with Griffin by some party liberals after he split with them on busing and other rights issues in 1972.

In addition, Domenici said yesterday that he believed five or six of the eight GOP freshmen went for Baker, turning the tide at the last moment.

Griffin said later, "It's difficult to get a head count on his kind of thing. Howard is a very able leader."

In the Democratic causus there were several changes in key staff personnel. J.S. Kimmitt, 58, for the past 10 years secretary to the Democratic majority causus under Mansfield, defeated Francis R. Valeo, 60, for the post of secretary of the Senate by a 40-to-21 vote. Valeo, a Mansfield protege, had held the Senate post for the past 10 years. The job pays $43,600 a year and involves supervision of about 200 per sons, including the Senate budget officer, various clerks, and the public an political records offices, libraries and document rooms, with an overall payroll of $3.3 million a year. Kimmitt's name was put forward by Gaylord Nelson (D-Wis.), Walter Huddleston (D-Ky.) and others.

To replace Kimmitt as secretary to the majority, the caucus elected Byrd aide James Duffy, former chief counsel of the Elections and Privileges Subcommittee.

In naming Byrd, the Senate takes as leader a man brought up in dire poverty, who joined the Ku Klux Klan as a youth, never got a college education until he came to Washington as a member of Congress, and has gradually moved to the center of the party over the years. The majority leader is the most powerful man in the Senate because he decides which legislation shall come up an dwhen, and thus holds life-and-death power over it.

Unlike the easygoing Mansfield, Byrd is much more a floor activist who openly pushes things the way he wants them to go. He is famous for his floor duels with Sen. James B. Allen (D-Ala.) when Allen is obstructing some legislation Byrd wants to bring to passage.

On the floor yesterday, Byrd's first moves were to introduce resolutions to tighten the filibuster rule to prevent dilatory action and to refer a major proposal an committee reorganization to the Rules Committee. He obtained orders to report back the filibuster resolution to the floor in 90 days and the committee reorganization proposal by Jan. 19 - the first steps in what could be two major changes in Senate procedure.