Robyn Beck/Agence France-Presse â Getty Images  Mr. JacksonÂ at the Democratic National Convention in Denver in 2008. Mr. Jackson resigned his House seat on Wednesday.  Mr. Jackson, 47, has been treated for bipolar disorder, and cited ill health as his reason for leaving. He also acknowledged in a resignation letter a continuing federal criminal investigation into the possible misuse of campaign funds and said, for the first time publicly, that he was cooperating with investigators.  “For 17 years I have given 100 percent of my time, energy and life to public service,” Mr. Jackson, a Democrat, wrote in the letter to 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.”  The timing of the resignation — 15 days after voters re-elected Mr. Jackson a 10th time to represent a district on Chicago’s South Side and southern suburbs — irked some here, who thought he should have made a decision before Election Day, rather than creating the need for a special election to replace him.  Around the nation, Mr. Jackson’s colleagues in Congress and in his own offices expressed sadness over his departure, but few seemed truly surprised. Mr. Jackson, who was once spoken of as a future candidate for the Senate or for mayor of Chicago or even president, disappeared from public view in June, seeking medical treatment in Minnesota and Arizona.  By this month, some leaders in Chicago, including Mayor Rahm Emanuel, had publicly suggested that Mr. Jackson needed to address his constituents, one way or another. Even on Wednesday, there was no official announcement of the resignation, only the letter. Mr. Jackson’s whereabouts was undisclosed. He did make private phone calls on Wednesday to tell some colleagues of his decision, and several described those talks as sorrowful. “He sounded in so much pain,” Representative Bobby L. Rush of Illinois said.  Still looming is the criminal inquiry into Mr. Jackson’s use of campaign funds.  “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,” Mr. Jackson wrote in his letter. “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.”  While Mr. Jackson did not cite the investigation as a reason for his departure, legal experts said his resignation might help lawyers argue for leniency, given that he has already paid a significant penalty: his job. A statement issued by his lawyers, including Dan K. Webb, a former United States attorney from Chicago, suggested that talks with prosecutors were under way. “We hope to negotiate a fair resolution of the matter, but the process could take several months,” the lawyers said.  Gov. Pat Quinn of Illinois pledged to set a date for a special election swiftly. By law it must take place within 115 days of that date. Already, numerous people appeared to be eyeing the seat. “Every other African-American politician I talk to on the South Side is thinking about running,” said Laura Washington, a political analyst here.  Some in Chicago have suggested that another member of the Jackson family — perhaps Mr. Jackson’s wife, Sandi, an alderman, or Mr. Jackson’s brother, Jonathan — may seek the job, but that remained uncertain on Wednesday. A spokesman for Mr. Jackson and his father did not return phone calls.  Colleagues praised Mr. Jackson’s tenure, noting his efforts to build a third airport in the Chicago area, his success in bringing nearly a billion dollars for projects in his district and his drive for an increase in the minimum wage.  “His service in Congress was marked by his eloquent advocacy for his constituents’ views and interests,” Representative Nancy Pelosi, the Democratic leader, said in a written statement.  Even before Mr. Jackson took a medical leave from Congress in June, his political star had fallen. In late 2008, he sought an appointment to fill the Senate seat that was being vacated by Barack Obama. As part of an inquiry into actions by former Gov. Rod R. Blagojevich of Illinois, now in prison for trying to sell that appointment, authorities said they learned that a friend of Mr. Jackson had offered campaign contributions to the governor if Mr. Jackson got the Senate job. A House ethics investigation opened, though Mr. Jackson denied wrongdoing.  On Nov. 6, Mr. Jackson won 63 percent of the vote in his Democratic-leaning district, though he had held no campaign appearances for months and was undergoing treatment at the Mayo Clinic in Minnesota on Election Day.  No victory party was held. Instead, Mr. Jackson spoke optimistically of his return to Congress in a statement.  “Once the doctors approve my return to work, I will continue to be the progressive fighter you have known for years,” his statement read. “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.”  Jennifer Steinhauer and Michael S. Schmidt contributed reporting from Washington, and Steven Yaccino from Chicago.
Representative Jesse L. Jackson Jr. won re-election this month despite an extended illness, a long absence from Congress and reports of looming investigations.