“My family legacy was never just about government service,” said Mr. Kennedy, who talked for more than two hours in an empty room at the Cannon House Office Building, where John F. Kennedy also worked as a House member from 1947 to 1953. “It was about giving back, and the branding of President Kennedy’s call for Americans to give back to their country.”  And yet it was politics that made the Kennedys a de facto royal family, giving them a vein of power in Washington that spanned generations. The Kennedys have been woven prominently through the political and social history of the last half-century, from the assassinations of John and his brother Robert, to Edward’s 1969 car accident on Chappaquiddick Island, Mass., that killed Mary Jo Kopechne, to the 1999 plane crash that killed John F. Kennedy Jr.  Recent forays into politics by other family members, like Caroline Kennedy’s brief run for the Senate in New York in 2009, have also fascinated the nation.  “It’s not as if a Kennedy presence in Washington is an indispensable ingredient for the survival of the republic,” said Ross K. Baker, a political science professor at Rutgers University. “But for people whose memories harken back to the earlier Kennedys, especially as American politics got more fractious and contentious, there was something reassuring about the element of continuity.”  Mr. Kennedy was 21 when he was elected to the Rhode Island House of Representatives in 1988, winning on his name alone. He never considered a life outside politics, he said, because he was so intent on emulating his father.  But he always struggled in the legislative shadow of one of the most influential senators in history. The younger Kennedy had his own signature achievement with a 2008 law that requires equal insurance coverage for treatment of mental and physical illness, and he became a strong proponent of removing American troops from Afghanistan. In recent months, he has advocated more research and treatment for veterans suffering from traumatic brain injury. Still, he was as well known for his family name and brushes with addiction as for his legislative work.  “Whereas his uncles and father were people whose footprints were indelible on the terrain of American politics,” Mr. Baker said, “Patrick was not.”  Other Kennedys may yet enter politics — Victoria Reggie Kennedy, Edward Kennedy’s widow, is seen as a possible Democratic Senate candidate from Massachusetts, and Joseph P. Kennedy III, 30, a grandson of Robert F. Kennedy, briefly considered running for an open House seat there this year — but to date, most of Patrick Kennedy’s cousins have pursued different kinds of public service.  Timothy Shriver, a son of Eunice Kennedy Shriver, runs the Special Olympics, for example, and Robert F. Kennedy Jr. is an environmental activist.  “I know it fits some narrative that, ‘Oh, I’m the last Kennedy,’ ” Patrick Kennedy said, his tone verging on sardonic. “But any one survey of what my family is doing out there in a million different ways fits with my family legacy.”  His way of giving back, Mr. Kennedy said, would be to continue as an advocate for ending the stigma of mental illness. He will draw attention and resources to brain research, he said, in hopes of improving how disorders from addiction to Parkinson’s disease are treated and understood.  His interest is personal, not least because he was treated for cocaine addiction as a teenager, was given a diagnosis of bipolar disorder after he got to Congress in 1994, and then became addicted to painkillers. In 2006, he crashed his Mustang convertible into a barricade outside the Capitol in the middle of the night, and then went public with his addiction and sought treatment.  He is planning to detail his struggles in a memoir, “Coming Clean,” to be released by Houghton Mifflin Harcourt late next year.  “Ultimately, I see telling my own story as a more palatable way to get out the story of the neuroscience,” Mr. Kennedy said. “I don’t want to be talking about salacious details for the purpose of salacious details, but for the purpose of fitting it into a context to describe a bigger story.”  Mr. Kennedy said he was closing down his campaign committees and not keeping any campaign money. He might keep an office in Washington, he said, but would consider Rhode Island home.  After he leaves the Hill, his immediate goal will be organizing a brain research conference in Boston in May. Not by coincidence, it will be the 50th anniversary of President Kennedy’s speech to Congress proposing to send a man to the moon. He is enlisting scientists and sponsors, and stressing that the initiative could prove as historic as the race to space.  He has set up a Web site, www.moonshot.org, and singled out veterans as urgently needing the kind of scientific breakthroughs he envisions.  With his father’s death in August 2009 still an open wound, Mr. Kennedy said he also had a more personal goal.  “I lost the most important person in my life,” he said. “So I’m looking forward to developing those emotional relationships with others, because there’s more to life.”  Norman J. Ornstein, a political scientist at the American Enterprise Institute, said that while Mr. Kennedy’s departure was minor in the scheme of things, the fact that he and his father were being replaced as the only father-son team in Congress by Representative Ron Paul of Texas and Senator-elect Rand Paul of Kentucky, who hail from the libertarian Tea Party wing of the Republican Party, was indicative of “the kind of sea change we’re going through” on Capitol Hill.  “To go from the Kennedys to the Pauls,” Mr. Ornstein said, “I would say that’s a pretty big difference.”  A version of this article appears in print on December 17, 2010, on Page A18 of the New York edition with the headline: The Capitol’s Last Kennedy. Order Reprints| Today's Paper|Subscribe
When Patrick J. Kennedy’s eighth term in Congress ends next month, no member of his family will hold national office for the first time since 1947.