When young Jim Eastland ran for the U.S. Senate for the first time in 1942, one of his campaign promises was to do everything he could to stop blacks and whites from eating together in the Capitol in Washington.

Eastland would make that pledge in many a courthouse square, then retreat to his car and joke with his companions: you know, he'd say, they really believed me!

According to his friends, the contrast between public positions and private good nature has been a cardinal feature of Eastland's 37-year career in the Senate. Over the years Eastland repeatedly took public positions that outranged liberal and moderate opinion and he blocked (or tired to) scores of civil rights bills, but through it all he kept a reputation as a good fellow among many of his Senate colleagues.

In recent years Eastland yielded much of his power as chairman of the Senate Judiciary Committee, and liberals on that committee never complained that the chairman was imperious or obstructionist.

Now those liberals will likely become the dominant influence on the Judiciary Committee when Eastland leaves the Senate in January. He confirmed his intention to retire yesterday. His successor as chairman of the committee is expected to be Sen, Edward M. Kennedy (D-Mass).

With Eastland's retirement, the Senate will contain just two old-time Southern segregationists, John C. Stennis (D-Miss) and Strom Thurmond (R-S.C.). Both of them have been wooing black voters for some years now, however - as Eastland would have, had he run for reelection this year.

It was easy to find a nostalgic conversation in the Capitol yesterday about the passing of old eras, but in fact they are already long passed.

In quiet recognition of the new times, Eastland has abandoned most of the old powers of his committee chairmanship, leaving subcommittee chairmen, including liberals like Kennedy and Birch Bayh (D-Ind.), freedom to do most of what they laked.

In a similar way, Eastland chose not to make much out of his ceremonial position as president pro tempore of the Senate. What preogatives that position did provide he generally shared with the majority leader and others.

Eastland has remained a living caricature of the southern senator: white hair, stooped shuffle, everpresent big cigar. He still speaks with one of the thickest accents in town, talking recently, for example, of the "Panymaw Canal." If he slips out of the stereo-type on any important point, it is his choice of whiskey - not bourbon, but Chivas Regal scotch. His ability to consume large quantities of it while retaining his composure is a Senate legend.

But the old fire went out years ago. Reading back now through yellowing clippings that record Eastland's earlier exploits brings back images of a very different senior Senator from Mississippi.

Time was when he would rise regularly in the Senate to rant against "mongrelization of the races," the "communistic" Supreme Court and other aspects of modern life that he found uncongenial.

As a leader of the Senate internal security subcommittee, Eastland joined in some of the most extravagant witch-hunting expeditions of the 1950's. In 1955 he urged the Senate to investigate the influence of "communistic groups" in the formation of the "modern scientific authority' upon which the Supreme Court relied in the school segregation cases."

The Supreme Court got Eastland down for years, though friends say he was delighted with the turn it has taken more recently thanks to President Nixon's appointments. In 1958 Eastland argued that the Earl Warren court "has been expanding its usurpation of the legislative field and purporting to make new law of general application which will be favorable to the communist position."

Earlier he deduced that the overwhelming number of the Supreme Court's decisions between 1943 and 1953 were "communist".

Eastland probably made his strongest mark on civil rights legislation, much of which he blocked for years and years.

In 1945 according to Robert Sherrill's "Gothic Politics in the Deep South," Eastland, on the campaign stump, boasted that he regularly broke the law as chairman of the Senate subcommittee that handled civil rights bills:

"You know," Sherrill quoted Eastland as saying, "The law says that the committee has to meet once a week. Why, for the three years I was chairman, that committee didn't hold a meeting. I didn't permit them to meet.

"I had special pockets put in my pants, and for three years I carried those bills around in my pockets everywhere I went and every one of them was defeated."

Between 1953 and 1965, 122 civil rights bills were introduced in the Senate, and only one of them got out of the Judiciary Committee. (Eastland became its chairman in 1956.)

Colleagues have differing recollections about Eastland's real personal interests in public issues. One who served with him for many years said yesterday, "He was always very parochial about things, I never heard of him having an original idea."

Others said they thought he really believed in the old witch-hunts, and certainly in segregation of the races.

All agree that Eastland's first love has always been his 5,000-acre plantation in Doddsville, Sunflower County, Miss. This is a cotton plantation in the richest part of the Mississippi delta country, now worth well over a million dollars.

One old friend said yesterday that he "spent too much time there, in my opinion, when he should have been in the Senate." A former aide said Eastland has regularly taken long weekends on the plantation for 45 weeks a year.

Eastland has collected hundreds of thousand of dollars in government subsidies for his cotton plantation - though, as liberal critics often noted, he never voted to help the nation's poor. When the government set a limit of $55,000 in federal assistance to any one farm, Eastland divided up his land among family members, and together they collected nearly $170,000 one year soon afterward.

"One thing on the plus side," a Louisiana journalist who has known Eastland for 40 years noted yesterday: "He and his family did take care of the black families on the farm . . . They did very well for the tenant farmers."

Another friend of Eastland in the Capitol yesterday produced a photograph of a substantial brick rambler with an air conditioner and garage. Eastland built houses like this for his black tenants, the friend said it was the sort of thing he would do for people he knew.

He also did political favors for colleagues with whom he never agreed on issues. His support; for example, helped give Walter F. Mondale a seat on the Senate Finance Committee.

But the secret to Eastland's benign personal reputation may have been his lack of personal pomposity. He has never been known to throw his weight around or claim rank.

Nor has he been a grandstander, particularly in recent years. "He's no Bilbo," one admirer said yesterday. "He's not even a Stennis," who can get wound up on the Senate floor. When Eastland speaks out now - which is rare - he usually reads quietly from a prepared text.

The curious thing about his departure now, it seems, is that it won't make much difference. Associates and friends of Sen. Kennedy have been talking for the past day and a half about how he might capitalize on the chairmanship of Judiciary, and they are having a hard time imagining many radical departures.

In fact Kennedy will carry on much as he has in the last couple of years, hoping to use an improved podium to advantage, but without any expectations that the world's greatest deliberative body will be shaken by his new position,

Jim Eastland, as one Kennedy ally put it yesterday, wasn't the problem.