"The constituents of the Second District deserve a full-time legislator in Washington, something I cannot be for the foreseeable future," Jackson wrote. "Therefore, it is with great regret that I hereby resign as a member of the United States House of Representatives, effective today, in order to focus on restoring my health."
In the letter, which was also sent to Minority Leader Nancy Pelosi (D-CA) and the rest of the House leadership, Jackson indicated that he had "hoped and tried" to return to Washington "against the recommendations of my doctors."
"I know now that will not be possible," Jackson wrote.
The Illinois Democrat, who was diagnosed with bipolar disorder earlier this year, also acknowledged publicly for the first time that he is the target of a federal investigation.
"I am aware of the ongoing federal investigation into my activities and I am doing my best to address the situation responsibly, cooperate with the investigators, and accept my mistakes, for they are my mistakes and mine alone," Jackson Jr. wrote. "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."
Jackson, 47, the son of civil rights icon Jesse Jackson Sr., began serving in the lower chamber in 1995, and just this month easily won reelection to another two-year term, despite the reports of a federal criminal probe looking into his alleged misuse of campaign funds.
Immediately after Election Day, reports indicated that Jackson Jr. had entered plea deal discussions with federal investigators, and that prosecutors were reviewing evidence that his wife, Chicago Ald. Sandi Jackson, was complicit in the alleged use of campaign money to redecorate their Washington D.C. home.
The probe was apparently already underway when Jackson Jr. collapsed in the D.C. home on June 10. He then spent the next several months being treated for bipolar disorder, including stints at clinics in Arizona and Minnesota.
The public has heard almost nothing from Jackson Jr. himself since his collapse. In October, a reporter for The Daily encountered Jackson outside his D.C. home, and the congressman said in a brief interview that he was "not well." Later in the month, his campaign released a robo-call in which he said that "like many human beings a series of events came together in my life at the same time and they have been difficult to sort through."
Jackson Jr. had reportedly scheduled a conference call with some of his staff members for Wednesday morning, but it was subsequently cancelled. According to The Chicago Sun-Times, Jackson Jr. also planned to give a press conference, but a source told the newspaper that the congressman "couldn't stop crying so he couldn't give a press conference."
The same source said that Jackson Jr. has yet to reach a plea deal with prosecutors.
In response to a call from TPM, the FBI's Washington Field Office referred a reporter to the U.S. Attorney's Office for the District of Columbia, which declined comment on the matter.
Read Jackson Jr.'s full letter: