“I am distressed by it,” said former Representative Barney Frank, calling the primaries “a luxury we should not be indulging in” when Democratic donors and activists could be helping in the presidential race and competitive Senate contests in nearby Maine and New Hampshire.

Mr. Frank, whose seat is now held by Mr. Kennedy, was characteristically blunt when asked about his successor taking on Mr. Markey, his longtime colleague in the House.

“I was shocked Joe would do that, I think it’s a mistake,” he said, speculating that Mr. Kennedy was considering a run now because he does not “want the seat to eventually go to anybody else.”

Many Democrats here say they find the plotting by the genial Mr. Kennedy to be out of character. Yet these assumptions also ignore the cutthroat side of the Kennedy family: the congressman’s grandfather, Robert, and his great-uncle, Edward, both challenged sitting Democratic presidents. (Mr. Kennedy declined an interview request.)