Patrick speaks emotionally about his late father in the farewell video
Rhode Island congressman Patrick Kennedy has decided not to seek re-election, a move set to end one of the US's great political dynasties.
Months after the death of his father Senator Edward Kennedy, Patrick, 42, is due to announce he will leave the House of Representatives next year.
In a video to be aired shortly, he says his life is "taking a new direction".
Unless another family member stands for office, there will be no Kennedy in Congress for the first time in decades.
Family members or close associates of the Massachusetts family have served there since 1947, according to the Boston Globe newspaper.
Edward Kennedy's brothers John and Robert, both assassinated, served respectively as president and attorney general in the 1960s.
Mr Kennedy gave no direct reason for his departure but a close aide, Democratic fundraiser Mark Weiner, said his father's death had taken an enormous toll on him.
"It's tough to get up and go to work every day when your partner is not there," Mr Weiner told the Associated Press news agency.
"I think he just had a broken heart after his father passed away."
Elected at 21
Mr Kennedy has been in and out of treatment for substance abuse since crashing his car outside the US Capitol in 2006 but has been comfortably re-elected twice since then, after making mental health care his signature issue in Washington, AP notes.
"When I made missteps or suffered setbacks, you responded not with contempt, but compassion," he tells voters in the two-minute video, due to be broadcast on Sunday.
"Thank you for all the times you lifted me up, pushed me forward and filled my heart with hope."
He says he wants to continue working to help those with depression, addiction, autism and post-traumatic stress disorder.
Over images of himself and his father, he adds: "My father taught me that politics at its very core was about serving others.
"For two decades I've been humbled and honoured to represent the people of our state.''
Mr Kennedy was elected to the Rhode Island House of Representatives in 1988 at the age of 21, while still a student, and entered Congress six years later.
As well as his work on mental health issues, he is credited with being a star fundraiser for the Democratic Party and reports suggest he would have been well placed for re-election in 2011 had he chosen to stand.