The earthquake of southern Democratic opposition that yesterday swallowed the Supreme Court nomination of Robert H. Bork began as a tremor a month ago at a long table in the Senate dining room presided over by Sen. J. Bennett Johnston (D-La.).

Johnston, a three-term conservative with ambitions to become the Senate majority leader, found himself holding court with a circle of younger members while the Senate droned on in late-night session.

"This nomination is going to go down because people like you are going to vote against it, and you know why?" Johnston said, leaning forward and waving a finger at Sen. Richard C. Shelby (D-Ala.), a freshman conservative who had voted as a House member against extending the Voting Rights Act and creating the Martin Luther King Jr. holiday.

"You're going to vote against it because you're not going to turn your back on 91 percent of the black voters in Alabama who got you here," Johnston said. Then he pointed at other southern Democrats at the table and around the dining room, one by one, saying: "And I know how you're going to vote, and you, and you, and you."

Yesterday seven southerners announced their nay votes, mostly in the order that Johnston had pointed them out -- Shelby, John B. Breaux (D-La.), Wyche Fowler Jr. (D-Ga.), Bob Graham (D-Fla.), Lawton Chiles (D-Fla.), Albert Gore Jr. (D-Tenn.) and Jim Sasser (D-Tenn.). Before them had come Sens. Terry Sanford (D-N.C.), David H. Pryor (D-Ark.), Wendell H. Ford (D-Ky.) and a number of others.

Breaux, Fowler, Sanford and Shelby were elected in November with support from only a minority of whites and about 90 percent of blacks, a coalition that has become a fact of political life for all Democrats in the Deep South. Graham, who got 52 percent of the white vote, received 86 percent from blacks.

Although a range of factors here and at home led southern Democrats to line up against Bork, many yesterday cited their informal group talks with Johnston, the wily chairman of the Senate Energy and Natural Resources Committee and likely future candidate for majority leader.

"The younger southern members in particular gravitated toward Bennett," said Pryor, a close friend of Johnston's and a regular presence at his table. "He comes from northern Louisiana, which isn't known for its liberalism. His state has rough-and-tumble politics. He's been victorious and he's been defeated. Bennett knows his politics."

Johnston was officially undecided in those talks, but participants said he believed from the start that most southern Democrats would oppose the nomination. "It was a subject Bennett wanted to talk about and listen about at a time when most people were still hesitant about bringing it up," Pryor said.

Sen. Howell Heflin (D-Ala.), a former Alabama Supreme Court chief justice and member of the Judiciary Committee, was expected to lead the way for other southerners. But Heflin withheld his verdict until Tuesday's committee vote, and Johnston, in the void, emerged as the South's behind-the-scenes statesman.

"I don't know that I really deserve the credit," Johnston said with deliberate modesty yesterday. "I don't know if I want it either."

Explaining his reservations, Johnston pulled from his pocket an editorial from his hometown newspaper, The Shreveport Times, denouncing his statement last Thursday opposing Bork, the first by a southern Democrat.

"This self-described 'conservative,' " the paper said of Johnston, "is representing not his state's conservative interests but rather those of the liberal national Democratic Party. Thus does Johnston preach from all the moral high ground of a prostitute."

Breaux, a close ally of Johnston's and the state's junior senator, called Johnston's role "an act of courage" that will "definitely" win him loyalty if he seeks the majority leader's job, as he did unsuccessfully in 1986.

The freshman southerners who gathered around Johnston -- mainly Shelby, Breaux and Fowler -- felt vulnerable at first on the Bork vote because they represent heavily conservative constituencies generally loyal to President Reagan. Yet each owed his narrow election to near-unanimous support from traditional Democratic constituencies opposed to Bork -- black voters, labor and women.

A vote either way seemed potentially suicidal.

But, as Shelby recalled, Johnston in the late-night talks underlined an impression beginning to form among the freshman southerners: the expected groundswell of support from southern conservatives wasn't taking shape, while the opposition was much wider than expected, reaching beyond liberals.

Several of the southerners called the ambivalence cultural, a response to Bork's testimony that he had been a socialist as a youth and had even attended a Communist Party meeting as a teen-ager. That Bork does not practice organized religion also weighed against him in Bible Belt states. "This was not Jerry Falwell, after all," Breaux said.

The freshman Democrats also had reason to send a message to Reagan, who campaigned against all of them in 1986.

"While the president and my opponent were basking in the limelight, raising millions of dollars at fund-raisers," Shelby said yesterday, "I was out working the plant gates, mainly in the cold and rain with my collar turned up, asking people one by one for their vote."

While counting on the southern Democrats to support Bork, the White House virtually ignored them. Breaux said he received his first call from the White House after Johnston's announcement last Thursday. Shelby said he was to see White House official Kenneth Duberstein last Friday morning, but by then he already had decided which way he was going.

"He called me {early Friday} and asked me should he come on up," Shelby said. "And I told him he shouldn't."

Staff researcher Michelle Hall contributed to this report.