IN LATE DECEMBER OF 1968, Sen. Edward Kennedy, then seeking to replace Sen. Russell Long as the Senate Democratic whip, paid a courtesy call on chairman James Eastland of the Senate Judiciary Committee, Sunflower County, Miss., and a supporter of Sen. Long. Sen. Eastland, the story goes, told the Massachusetts challenger: "Kennedy, I don't know what you're talking to me about. 'There ain't no vacancy."

Former Sen. Eastland will not attend Sen. Kennedy's formal announcement of his presidential candidacy in Boston this morning. But chairman Eastland's earlier observation will be there, especially in the minds of many who saw Sen. Kennedy's performance on CBS Reports last Sunday night. Even the fiercest Kennedy partisan would have to concede the inadequacy of the senator's answer to Mr. Mudd's question, "Why do you want to be president?": "The reasons I would run are because I have great belief in this country, that is -- there's more natural resources than any nation of the world, there's the greatest educated population in the world . . . and the greatest political system in the world; it just seems to me that this nation can cope and deal with the problem in a way it has done in the past." After serveral more rambling sentences, Mr. Kennedy concluded his statement about his potential presidency in this way: "And I would basically feel that it's imperative for the country to either move forward, that it can't stand still or otherwise it moves backward." Forward, backward -- at a time of seemingly permanent double-digit inflation and reappearing gasoline lines, automobile transmission analogies seem particularly inappropriate, not just inadequate.

So Mr. Kennedy's announcement speech will be a significant political event. He is the candidate of change and the acknowledged leader in all public opinion polls. In his speech, he has an obligation to tell us how he differs from President Carter on questions of inflation, energy and managing the economy. For many, Mr. Kennedy will be defining his campaign. Who are the villains that Mr. Kennedy, as president, would bring to the bar of justice? Who are the victims of the federal government's indifference or interference that Mr. Kennedy, as president, would help first? What are the differences Mr. Kennedy has with President Carter concerning the direction and emphasis of this nation's foreign policy and national defense?

Action verbs will not do today in Boston or tomorrow in Manchester. Sen. Kennedy, as the consensus leader of a large pack and the pack and the principal challenger to an incumbent president, has both a special opportunity and a special obligation to help set the national agenda and influence the national debate.