IF EVER a man in public life had the right to say, "I told you so," it is Daniel Patrick Moynihan, Democratic senator from New York.

Twenty-one years ago this June, Moynihan, then a 37-year-old assistant secretary of labor, wrote a confidential government document called "Report on the Negro Family." It told of the breakdown of the black family.

He was pilloried and berated, accused of racism and sloppy research. At a convocation of black leaders called by Lyndon Johnson in November 1965, Moynihan was shunned. "I've become a non-person," he told a friend as he wandered through the hotel lobby looking for someone to talk to.

At the time, a 31-year-old Texas preacher named Bill Moyers was a White House staff aide. Moynihan showed him the report. Moyers thought it was a proper theme for a presidential address. Johnson agreed and, in a famous speech at Howard University, spoke about the crisis that could wipe out the civil rights gains that were then being won. The speech was so well received that Moyers advised Johnson to publish the Moynihan report.

"All hell broke loose," he recalls now.

Moynihan was savagely attacked by black leaders. James Farmer accused him of "a new brand of bigotry." Andrew Young said that Moynihan was all wrong, that he did not understand the "extended family" tradition of blacks.

Looking back now, Moyers says he understands why blacks fought off the report so fiercely.

"The Civil Rights Act had been passed, but the Voting Rights Act was still going through Congress," he said. "Martin Luther King called me and said that the awful problems disclosed in the report would be attributed to 'innate Negro weakness and used by racists to justify opposition to voting rights.'"

A conference -- "To Fulfill these Rights" -- had been called by Johnson in his Howard speech. It became a sort of lynching party against the Moynihan report. The subject of the Negro family, which was to have been the centerpiece of the gathering, was dropped from the agenda. Moynihan left town.

Now 2l years later, Moyers, a CBS commentator, has updated the Moynihan report. In a powerful two-hour documentary called "The Vanishing Family -- Crisis in Black America," he has put faces and voices to the statistics. He has explored the lives and feelings of unmarried black teen-age parents. They talked to him with stunning candor.

We meet Clarinda, age 16, who gave up high school when she had Shaquna; Timothy, who is proud of his prowess as a "baby-maker" -- he has six children by four women and supports none of them, and Alice, who at 23 has three children and can't say she won't have more because that's what she said after she had her first one.

In the panel discussion among prominent blacks that follows this devastating excursion into the sub-world of Newark streets, there is none of the rage of rejection that greeted Moynihan's prophetic account. One of the participants, Eleanor Holmes Norton, made her peace with Moynihan's facts some time ago. She said at a black forum in 1984 that blacks had made a mistake by looking at "the messenger, Moynihan, and not the message."

Harvard Prof. Glenn C. Loury says, "We paid a tremendous price for forcing the issue off the docket."

Things have gotten more than twice as bad since 1965. Then the rate of black illegitimate births was about 25 percent. Today it is 59 percent. Single parent families have doubled. "Today," Moyers says on the documentary, "black teenagers have the highest pregnancy rate in the industrial world, and in the black inner city, practically no teenage mother gets married."

"There's not too much to be encouraged about," Moyers said in an interview. He spent 10 months on the show; its two producers, Ruth Streeter and Kate Roth, both white, spent seven months in a Newark ghetto. Moyers consulted every known expert. The most devastating data came from University of Chicago Prof. William Julius Wilson, who told him that if the trend continues, "by the end of the century, 70 percent of black families will be headed by single women and 30 percent of black men will be unemployed."

Sex in the ghetto, Moyers found, is "as common as having a cup of coffee." The teen-age mothers confided that it was unsatisfying. "They don't care about the men -- they do nothing for them but sex."

No stigma is attached to having an illegitimate baby. "It's accepted, seen by some as a badge of honor, a moment of power and accomplishment."

Moynihan's report has been born again. This time it will not be kicked to death.

The senator declines to claim vindication. Blacks may never forgive him for the report, or for the "benign neglect" memo he wrote while in the service of Richard Nixon. But they will accept what they angrily rejected 20 years ago.