UPDATED: 11/21/2012 - 5:04 p.m.
CHICAGO (CBS) -- Congressman Jesse Jackson Jr. has resigned from Congress, citing health reasons and an intensifying federal investigation into the use of his campaign funds.
He notified staff, supporters and some lawmakers this afternoon. He was diagnosed with bipolar disorder this summer.
His resignation letter has been sent to House Speaker John Boehner.
"My health issues and treatment regimen have become incompatible with service in the House of Representatives," Jackson wrote. "Therefore, it is with great regret that I resign as a member of the United States House of Representatives, effective today, in order to focus on restoring my health."
CBS 2 Chief Correspondent Jay Levine reports the decision was made this week, jointly by the Jackson family, with the congressman's father, Rev. Jesse Jackson Sr., presiding – as with all major Jackson family decisions.
No one would say where Congressman Jackson was on Wednesday, but given the family's longstanding traditions, it's likely he's in Chicago, with the family for the Thanksgiving holiday.
For a man who wanted to be a U.S. Senator, considered running for mayor of Chicago, and was even thought of as possible Presidential material one day, it has been a sudden and unexpected end to Jackson's political career.
House Democratic Leader Nancy Pelosi, after speaking with both the former Congressman and his father Wednesday afternoon, said she accepted the news with great sadness.
Jackson's decision came following two weeks of mounting calls for him to come clean with voters, following revelations that his defense lawyer, former U.S. Attorney Dan Webb, was involved in plea bargaining with authorities who'd been investigating Jackson's alleged misuse of campaign funds.
Jackson's resignation letter also included his first public acknowledgement of the federal probe.
"During this journey, I have made my share of mistakes. I am aware of the ongoing federal investigation into my activities and I am doing my best to address the situation responsibly," he wrote.
A statement from Jackson's lawyers, including Webb, said, "Mr. Jackson is cooperating with the investigation. We hope to negotiate a fair resolution of the matter but the process could take several months. During that time, we will have no further comment and urge you to give Mr. Jackson the privacy he needs to heal and handle these issues responsibly."
There have been news reports the feds are also investigating Jackson's wife, Ald. Sandi Jackson (7th) in connection to possible misuse of campaign funds, but Jackson's letter clearly seeks to absolve her of any wrongdoing.
Congressman Jackson easily won re-election earlier this month, despite not appearing in public since he took a medical leave of absence in June.
Though for several weeks now, it has no longer been a matter of if he would resign, but when.
Those who've met and spoken with Jackson recently said the congressman was nowhere near ready to return to work now.
And there were serious doubts that, given his bipolar disorder -- diagnosed by the Mayo Clinic this summer -- he would ever be ready to subject himself to the stresses and pressure of elected office.
Any plea agreement, negotiated by Webb and other Washington lawyers, had been expected to include Jackson's resignation. It's also expected he'll be allowed to keep his federal pension, but will be required to repay any campaign funds spent for personal use. At least some jail time would appear likely.
There's no word yet on whether that might either include or be replaced by hospitalization for a condition known to cause the kind of behavior that triggered the scrutiny of both his official duties and personal life, including improper, irrational spending and risky sexual misconduct.
Gov. Pat Quinn now has five days to announce a date for a special election to fill Jackson's seat. That election must occur within 115 days of the resignation.
When told of the news, fellow Illinois Congressman Danny Davis said, "I'm saddened to hear the news. I'd hoped that Congressman Jackson would be able to return to his duties. He is one of the most able members of Congress because of his intellect and effectiveness. I will be sorry not to have him in Congress. It will be a great loss to the nation."
North Side Congressman Mike Quigley said he's known Jackson for about 12 years. He said he isn't surprised to learn Jackson has decided to step down, given the combined pressures of his health issues and the federal investigation.
LISTEN: WBBM Newsradio's Julie Mann Reports
Quigley said he hasn't heard anything from Jackson since he began his leave of absence from Congress in June, but said Jackson was a responsive Congressman when he was on the job.
"When Congressman Jackson was there, he rarely missed a vote. He was very conscientious about such things. He cared a great deal about his constituents, and my reaction was that he liked being a congressman," Quigley said.
He said Jackson's resignation reminds him that people in elected office need to be accountable, and serve in the most transparent way possible.
The news of Jackson's resignation caught constituent Carl Jones by surprise. He was walking past the former congressman's closed office at 71st and Yates.
"He was like family," Jones said. "I knew his father well, so I was just surprised. I feel for his wife and his family."
LISTEN: WBBM Newsradio's Mike Krauser Reports
Second District resident Kenneth Jackson, no relation, said he believes Congressman Jackson has done "a fairly decent job" representing the 2nd Congressional District, but due to the his mental health issues, "I think he needs to take some time off to work on those issues, and then maybe try it again later."
As for speculation that Jackson is stepping down now to protect his pension before a possible plea deal in connection to the federal investigation of his campaign finance activities, Kenneth Jackson said, "I think that's just a political statement. I think, basically, his emotional and mental problems come first, and then the other side, as far as his trials and tribulations. That's another matter. I think he's a great guy."
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