Representative Jesse L. Jackson Jr., who has undergone months of treatment for bipolar disorder and is under federal investigation, resigned from Congress on Wednesday.
"For seventeen years I have given 100 percent of my time, energy and life to public service," Mr. Jackson wrote in a resignation letter to House Speaker John A. Boehner. "However, over the past several months, as my health has deteriorated, my ability to serve the constituents of my district has continued to diminish. Against the recommendations of my doctors, I had hoped and tried to return to Washington and continue working on the issues that matter most to the people of the Second District. I know now that will not be possible."
In his hometown, Chicago, and in Washington, the decision puts an end to five months of speculation about the political future of Mr. Jackson, a Democrat and the son of the civil rights leader bearing the same name, who had vanished from Congress and public view since June.
Still uncertain for Mr. Jackson is how a criminal investigation into his possible use of campaign funds on purchases for his home may turn out.
In his letter of resignation, Mr. Jackson addressed the investigation directly, saying: "I am aware of the ongoing federal investigation into my activities and I am doing my best to address the situation responsibly, cooperate with investigators, and accept responsibility for my mistakes, for they are my mistakes and mine alone. None of us is immune from our share of shortcomings or human frailties and I pray that I will be remembered for what I did right."
Even before word of Mr. Jackson's decision began spreading, political figures in Chicago were scrambling for the seat, which Mr. Jackson had occupied for nearly two decades and is now expected to be filled through a special election.
On Election Day a few weeks ago, Mr. Jackson won 63 percent of the vote in his district on Chicago's South Side and its southern suburbs, even as he remained under treatment for bipolar disorder at the Mayo Clinic in Minnesota. By a week after Election Day, Mr. Jackson had left the Mayo Clinic, officials there said.
Mr. Jackson, who was once talked about as a future United States senator or a mayor of Chicago, has seen that vision unravel in the past few years. Around the nation, Mr. Jackson's colleagues in politics reacted with sadness and complimentary sendoffs, if not shock.
"His service in Congress was marked by his eloquent advocacy for his constituents' views and interests," Rep. Nancy Pelosi, the Democratic leader, said in a written statement released after she had spoken to Rep. Jackson and his father, the Rev. Jesse L. Jackson Sr., the leader of the Rainbow/PUSH Coalition.
Separate from the federal investigation into Mr. Jackson's campaign fund spending, a House ethics investigation has continued over Mr. Jackson's actions in 2008, when he sought an appointment to fill the Senate seat that was vacated by Barack Obama. As part of an inquiry into actions by former Governor Rod R. Blagojevich of Illinois, who is now serving prison time for trying to sell the appointment to Mr. Obama's Senate seat, federal authorities said they learned that a close friend of Mr. Jackson had offered large campaign contributions to Mr. Blagojevich if Mr. Jackson was given the Senate job. Mr. Jackson, who has said he was unaware of his friend's offers, has acknowledged hoping to join the Senate but has denied wrongdoing.
In early 2012, even as that ethics investigation went on, Mr. Jackson faced a primary challenger, former Representative Debbie Halvorson, and ran one of his most active, aggressive campaigns in many years. In the March primary, he soundly defeated Ms. Halvorson with more than 70 percent of the vote.
But by June, Mr. Jackson dropped out of public view. His office initially reported that he was suffering from exhaustion, but his representatives eventually announced that he had been hospitalized for what is known as "bipolar II disorder," a condition that was apparently complicated by weight loss surgery he had undergone in 2004, his office said, changing the way his body absorbs substances including medication.
Around Labor Day, he returned to his Washington home to recuperate, his aides said, but remained absent from work and the campaign trail, and not long before Election Day, he returned to the Mayo Clinic for additional treatment. Meanwhile, reports emerged about the federal investigation into Mr. Jackson's campaign funds, including accusations that money had gone to decorating Mr. Jackson's home in Washington, D.C.
Voters, it seemed, did not care, particularly given the strong Democratic leanings of his district and a group of little-known challengers for his seat. While Mr. Jackson's margin of victory was not as large as it has been in some years, it was decisive; his 63 percent of the vote compared with 23 percent for Brian Woodworth, a Republican, and about 13 percent for Marcus Lewis, an independent.
Standing on the driveway of his home in Chicago's South Shore neighborhood recently, Scott Onque, 49, said he had voted for Mr. Jackson because he was the only Democratic candidate, and because no accusations against him have been proved. "We'd rather go with what's tried and tested than what we don't know," he said.
The congressman's illness and absence from the district was a separate issue. "I think we need to give a person a chance to get better," Mr. Onque said, adding, "It's still his show."
Still, as reports of investigations continued to emerge, some in the area said doubts had begun to creep in. "I think it's misguided loyalty," Bridgette Bynum, 45, said of Mr. Jackson's election victory. "Many people feel sorry for him or something," she added.
In recent days, many around Mr. Jackson -- including his Congressional aides and political strategists -- said they knew little about the legal circumstances swirling around him. His wife, an alderman in Chicago, did not attend a City Council meeting this month, and no traditional election night event was held to celebrate his victory.
Instead, Mr. Jackson's office issued a statement of thanks on Nov. 6, as the polls showed him winning a 10th election. Mr. Jackson made no mention of legal issues and spoke optimistically of returning to Congress.
"Every day, I think about your needs and concerns," Mr. Jackson said in the statement. "Once the doctors approve my return to work, I will continue to be the progressive fighter you have known for years. My family and I are grateful for your many heartfelt prayers and kind thoughts. I continue to feel better every day and look forward to serving you."
In recent weeks, many in Chicago had begun speculating about Mr. Jackson's future -- and about possible replacements in Congress. Under Illinois law, Gov. Pat Quinn has five days to set an election date to fill Mr. Jackson's seat. That election must take place within 115 days of the governor's announcement.
Not long after Mr. Jackson's re-election, Mayor Rahm Emanuel suggested that he needed to end his silence soon.
"I do think it's essential that the congressman, who has been out for a while, begin a conversation with his constituents about his intentions," Mayor Emanuel said on Nov. 15. He had sounded a more forgiving tone when it came to Alderman Jackson, who had missed several meetings, including one in which the city's new budget was voted on. "All of us know that this is a unique moment in time for her as both a spouse and a parent, let alone as an alderman," Mr. Emanuel said. "And she'll make those decisions."
Publicly, leaders for months deemed it premature to think about who might replace Mr. Jackson in Congress if he did not return to work. But behind closed doors, the jockeying appeared to be well under way. Alderman Carrie M. Austin, an Illinois Democratic Party committeewoman from Mr. Jackson's district, said party leaders in the state have been quietly considering possible replacement candidates.
Among those said to be considering a run for the seat: Samuel E. Adam Jr., a defense lawyer who represented former Governor Blagojevich in the corruption case that had raised questions about Mr. Jackson in 2008. "Whatever you want to say about Congressman Jackson, certainly he's done some things in the Second District, but ever since the Blagojevich stuff came out everything has been focused on him and not the people he's supposed to be representing," Mr. Adam said in an interview this month .
Jennifer Steinhauer contributed reporting from Washington, and Steven Yaccino from Chicago.
This article originally appeared in The New York Times.