Patrick Kennedy has decided not to seek re-election after eight terms in Congress, saying his life is "taking a new direction" just months after the death of his father and mentor, Senator Edward Kennedy.
The Rhode Island Democrat taped a message announcing his decision, which is to be aired on US television on Sunday night.
"Now, having spent two decades in politics, my life is taking a new direction, and I will not be a candidate for re-election this year," Kennedy says.
The decision comes less than a month after a stunning upset by Republican Scott Brown in the race for the Massachusetts Senate seat his father held for almost half a century.
Last week, as Brown was sworn into the seat, Patrick Kennedy called Brown's candidacy a "joke" and predicted Brown would betray his union supporters.
Kennedy did not give a reason for his decision, but he began the message by saying it had been a difficult few years for many people, then segued into the death of his father.
"Illness took the life of my most cherished mentor and confidante, my ultimate source of spirit and strength," he said, as a black-and-white photo of him as a boy sailing with his father appeared on the screen.
"From the countless lives he lifted, to the American promise he helped shape, my father taught me that politics at its very core was about serving others."
Mark Weiner, a major Democratic fundraiser in Rhode Island and one of Kennedy's top financial backers, said he had spoken with Kennedy about his decision, and that his father's death had taken an enormous toll.
"It's tough to get up and go to work every day when your partner is not there," Weiner said. "I think he just had a broken heart after his father passed away."
Kennedy said in his ad that he remained committed to public service, and he thanked Rhode Island voters for supporting him through ups and downs.
He has been in and out of treatment for substance abuse since crashing his car outside the US Capitol in 2006. Still, he has been comfortably re-elected twice since then, after making mental health care his signature issue in Washington.
"When I made mis-steps or suffered setbacks, you responded not with contempt, but compassion," he said. "Thank you for all the times you lifted me up, pushed me forward."
Kennedy was not specific about his plans, but said he would continue to fight for issues including on behalf of those suffering from depression, addiction autism and post-traumatic stress disorder.
As a member of the powerful House Appropriations Committee, Kennedy has made sure federal dollars are sent to his state.
Democratic Representative Jim Langevin, Rhode Island's only other House member, had spoken with Kennedy and said he would miss him in Congress.
"Patrick is a true public servant and passionate fighter who made a real difference for the people of our great state," said Langevin, who served with Kennedy in the state House of Representatives.
Kennedy was elected to that position in 1988 at age 21 while still attending Providence College, then was sent to the US House of Representatives in 1994.
Kennedy also has been a financial boon to the Democratic Party, drawing people to fundraisers nationwide, and he once served as chairman of the Democratic Congressional Campaign Committee.
In Congress, Kennedy has pushed for greater mental health care coverage, citing his own struggles with depression and addiction. His mother, Joan Kennedy, has been through several alcohol treatment programmes.
Kennedy (42) has never married, and friends have said his personal life had taken a back seat to his career.
Still, until recently, Kennedy appeared committed to running again.
The only Republican in the race, state representative John Loughlin, has been working with Brown's campaign team, the Shawmut Group, and was raising money. But Kennedy was heavily favoured to win the race: Registered Democrats outnumber Republicans more than 4 to 1 in Rhode Island. Loughlin has little statewide recognition, and Kennedy had four times as much campaign cash on hand coming into the year.
He told reporters shortly after Brown's win in January that he wasn't worried about Loughlin, saying "bring it on."
Weiner said Kennedy was not afraid of losing the election.
"Winning or losing had nothing to do, I'm sure, with his decision," he said.
No Democrat has entered the race for the seat.
Kennedy has had a difficult time in the public spotlight, with a number of high-profile troubles.
Most recently, he engaged in a protracted public spat with Providence Roman Catholic Bishop Thomas Tobin over health care reform and Kennedy's support of abortion rights.