We all know that, ”Lonesome Dove” aside, long miniseries are supposed to be out of fashion. This one, about the early years of a family that has been subjected to a lot of TV-movie exploitation, has been in the works for years, and was cut almost in half from its original 11 hours. So it seemed that The Kennedys of Massachusetts would have to be a botched job, a dreadful drag.

Instead, over its three days, the miniseries delivers the week’s most affecting drama and best acting, with William Petersen and Annette O’Toole offering remarkable detailed, unsparing portrayals of Joe and Rose Kennedy, the couple who built Camelot for their kids.

Based on Doris Kearns Goodwin’s book ”The Fitzgeralds and the Kennedys,” the movie is like ”The Brady Bunch” with brains and Boston accents. We watch the youthful Joe Jr., John, Rosemary, Kathleen, Eunice, Patricia, Robert, Jean, and Ted grow up and meet their fates. But ”The Kennedys of Massachusetts” stops when John becomes President; here, it’s the parents who matter.

Until now, William Petersen has pursued a career as a thoughtful action-hero in such movies as ”To Live and Die in L.A.” and ”Manhunter”; his complex, devious, grinning family-man Joe Kennedy is thus a complete departure for him, and so utterly successful that Petersen is at first unrecognizable in the role.

Annette O’Toole, on the other hand, has managed to elude stardom by taking unsympathetic roles in such films as ”One on One” and ”The War Between the Tates.” Her grim, unyielding Rose Kennedy is therefore entirely in keeping with O’Toole’s brave lonely career.

Much of the credit for the artistic success of ”The Kennedys of Massachusetts” must go to director Lamont Johnson, who does something rarely attempted in historical TV movies: He leaves in the politics. Joe and his obstreperous brood discuss isolationism, prejudice against Irish Catholics, and New Deal democracy around their dinner table. Even if you don’t believe the conversation could possibly have been as lively and articulate as it is here, it’s still solid, thought-provoking chatter. A-