It had to come sooner or later. The very experienced political forces behind Sen. Edward M. Kennedy (D-Mass.) obviously decided it was in their candidate's best interest that it come sooner.

"do you think, Senator," asks Roger Mudd, "that anybody will ever fully believe your explanation of Chappaquiddick?"

And with that, the television presidential campaign of 1980 is off to an official, bombshell start, as "CBS Reports on "Teddy," Sunday night at 10 on Channel 9. How the public reacts on Channel 9. How the public reacts to this unusual, intimate, compelling hour with Kennedy could have tremendous impact on the candidacy he plans to announce next week.

This program may in fact have as much effect on the early days of the campaign as the Nixon-Kennedy TV debates had on the late days of the 1960 presidential election. It seems UNTHINKABLE THAT THIS "CBS Reports" won't be looked back on as a political milestone, one way or another. It may be one for TV journalism as well.

ABC News rushed its own quickie interview with Kennedy onto the air Thursday night on "20/20" with additional excerpts seen yesterday on "Good Morning, America" and "World News Tonight." Tom Jarriel asked many questions similar in bluntness to Mudd's, but the CBS program is far more comprehensive and authoritative.

It begins with images for the most part benign or irresistibly emotional --the Kennedy brothers, American princes, alive and vital. But the montage includes a later shot of their grieving mother and then an Edgartown, Mass., police official referring to the tragedy at Chappaquiddick.

The hour is dominated by two major interviews, one filmed outdoors at Cape Cod, the other in Kennedy's Capitol Hill office. In both interviews, the Senator appears at points uncomfortable, faltering, almost dazed, as compared to the dynamic figure seen charging down a Senate hallway, moving a crowd of laborers to cheers, or, in a vision of iconographic deja vu, piloting a sailboat with his son.

Mudd is tough in his questioning, but he never resorts to the theatrics of a Mike "The Avenger" Wallace. In addition to the incendiary subject of Chappaquiddick, discussed at length, though without any new revelations, Mudd also asks about the danger of a third Kennedy assassination, about the status of Kennedy's marriage and about "published reports that link you to other women."

Kennedy's performance on this program will be analyzed ad infinitum in the weeks ahead. To some, he may indeed appear befuddled or uncertain, but to many others he will seem only vulnerable, haunted, and unexpectedly humble.

Many will undoubtedly look upon Kennedy's participation in this kind of questioning as itself an act of atonement, and the broadcast, coupled with the ABC footage, could have the effect of neutralizing Chappaquiddick as an issue by getting it out of the way early in the campaign.

But what this superbly done CBS Reports" also acknowledges and makes apparent once more is the incredible charismatic pull of the Kennedy name and legend. Even though Mudd refers to them sardonically, shots of Hyannisport, that shrine to the good life, and the sight of Kennedy kids tossing a frisbee or a football, still have incalculable popular allure, as do the Kennedyesque pronunciations like "agender," "ahsk" and "ahnswer." It all fits in with a myth we love.

If America longs for a movie star president again as much as some observers think, there is no question that one is available.

"teddy" was to have been shown next Wednesday but the air date was moved up -- largely through the efforts of Mudd himself -- to avoid equal time complications that would have arisen from Kennedy's announcement of his candidacy earlier that day. The program will benefit from the early evening lead-in of "60 Minutes" but will suffer from the incomparable competition of "Jaws," making its network debut on ABC.

(The irony may be peripheral, but "Jaws" was filmed on Martha's Vineyard and includes shots of the ferry to Chappaquiddick, the ferry Kennedy and Mary Jo Kopechne would presumably have taken if he had turned left that night instead of right).

However many millions of people see the program, it is bound to have considerable effect not only on the campaign to come, but on TV's political reporting to come as well.