Jackson has been in and out of the Mayo Clinic in Minnesota. The fall of Jesse Jackson Jr.

The once-promising political career of Rep. Jesse Jackson Jr. came to a crashing end Wednesday, when the Illinois Democrat announced he would resign his seat in Congress immediately amid treatment for mental illness, stories of marital infidelity and a pair of federal investigations.

The Illinois Democrat, the son of the civil rights icon the Rev. Jesse Jackson Sr., has been absent from the House since June 10, when he cast his last vote and then disappeared from public view, only to emerge later under treatment for bipolar disorder.


( PHOTOS: Jesse Jackson Jr.'s career)

Jackson Jr., 47, has been under federal investigation for alleged campaign finance improprieties, including reportedly using donor dollars to remodel his home and purchase personal gifts, a potential criminal violation.

In a two-page resignation letter sent to Speaker John Boehner (R-Ohio) on Wednesday, Jackson said he is “doing my best to address the situation responsibly, cooperate with the investigators, and accept responsibility for my mistakes, for they are my mistakes and mine alone.”

Jackson also said he is praying that he “will be remembered for what I did right.”

( Also on POLITICO: Jesse Jackson Jr.'s resignation letter)

Jackson has also been implicated — but never charged — in the scandal surrounding now-imprisoned former Gov. Rod Blagojevich’s attempt to sell President Barack Obama’s Senate seat.

Shortly after Obama won the White House in 2008, Jackson lobbied Blagojevich about being appointed to the vacant Senate seat. Jackson reportedly directed Raghuveer Nayak — an Illinois businessman — to tell Blagojevich that he would raise $6 million for the governor’s reelection campaign in return for the Senate appointment, according to a report by the Office of Congressional Ethics. Jackson denied the allegation.

Nayak was indicted June 20 by the Justice Department on a variety of federal tax and fraud charges, shortly after Jackson disappeared from Capitol Hill.

Nayak also paid for two round-trip airline tickets for a “social acquaintance” of Jackson’s, a Washington restaurant hostess, to fly to Chicago. In September 2010, Jackson issued a public apology for his interaction with Giovana Huidobro but never explicitly admitted to an extramarital affair.

Jackson’s defense team released a statement on Wednesday saying that he is “cooperating” with federal investigators, but noted it may take “several months” to work out a potential resolution of the case against him.

“Mr. Jackson is cooperating with the investigation. We hope to negotiate a fair resolution of the matter but the process could take several months,” said the statement from Reid Weingarten and Brian Heberlig of the law firm Steptoe & Johnson, and Dan Webb of Winston & Strawn. “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.”

Following his departure from the House five months ago, Jackson retreated out of the public eye and into the Mayo Clinic in Rochester, Minn., where he was treated for bipolar disorder, according to his doctors. Jackson briefly returned to Washington in mid-October but quickly left after reporters staked out his Dupont Circle home and he was spotted inside a D.C. bar.

Now, just days after winning reelection to his South Side Chicago seat with more than 60 percent of the vote, Democratic Gov. Pat Quinn must schedule a special election to replace Jackson. The seat will remain vacant until then.

In his resignation letter, Jackson touted many accomplishments of a 17-year career in the House, especially building “train stations, water towers and emergency rooms.”

“For seventeen years I have given 100 percent of my time, energy, and life to public service,” Jackson said in his resignation letter. “However, over the past several months, as my health has deteriorated, my ability to serve the constituents of my district 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.”

Jackson’s letter was also sent to Minority Leader Nancy Pelosi (D-Calif.), Majority Leader Eric Cantor (R-Va.), Minority Whip Steny Hoyer (D-Md.), Congressional Black Caucus Chairman Emanuel Cleaver (D-Mo.), Assistant Democratic Leader Jim Clyburn of South Carolina, incoming CBC Chairman Marcia Fudge (D-Ohio), Rep. Maxine Waters (D-Calif.) and fellow Chicago Democratic Reps. Bobby Rush and Danny Davis.

In a statement, Pelosi said it is with “great sadness” that she learned of Jackson’s decision to resign.

“His service in Congress was marked by his eloquent advocacy for his constituents’ views and interests,” Pelosi said in the statement. “Through his public statements and his writings, he presented a fresh perspective on how we work together to form a more perfect union. As he works to address his health, our thoughts and prayers are with him, his wife Sandi, his children as well as his parents. We are grateful to him and his family for their longstanding record of public service to our country.”

It is a tragic — even shocking — end for a once highly promising political career. At different times, Jackson considered running for Senate and toyed with the idea of campaigning for mayor of Chicago. He had a seat on the powerful, money-doling Appropriations Committee and was considered all but untouchable back home.

Jackson, whose wife, Sandi, is a Chicago alderman, was a close ally of Obama during his time in the Senate and run for the White House in 2008.

Jackson’s life — both politically and personally — was shaped in large part by his relationship with his famous father, a close associate of Martin Luther King Jr. The younger Jackson was exposed to national issues at an early age, and he met iconic African-American leaders and celebrities, including boxer Muhammad Ali and baseball star Jackie Robinson.

“Born in the midst of the voting rights struggle on March 11, 1965, Representative Jackson spent his twenty-first birthday in a jail cell in Washington, D.C. for taking part in a protest against apartheid at the South African Embassy,” reads Jackson’s official House biography. (Jackson Sr. played a high-profile role in U.S. protests against the apartheid regime.) “He also demonstrated weekly in front of the South African Consulate in Chicago. Representative Jackson was on stage with Nelson Mandela during his historic speech following a 27-year imprisonment in Cape Town.”

The younger Jackson is the second of five children and eldest son of the Rev. Jackson and Jacqueline Lavinia Brown. He graduated from North Carolina A&T State University in 1987 with a degree in business management. Three years later, he earned a degree in theology from the Chicago Theological Seminary. In 1993, he graduated from the University of Illinois College of Law.

Following a stint as secretary for the Democratic National Committee’s Black Caucus, Jackson became the national field director for the National Rainbow Coalition, founded by his father nearly a decade earlier. He was also a member of People United to Save Humanity, also founded by the Rev. Jackson in the 1970s.

In 1994, Democratic Illinois Rep. Mel Reynolds was indicted on charges of having sex with a 16-year-old campaign worker. Although Reynolds won reelection that November despite the scandal, he was convicted the following year of sexual assault, obstruction of justice and solicitation of child pornography. Reynolds resigned Oct. 1, 1995.

Jackson Jr., with his father’s backing, then won the special election to replace Reynolds. With his famous name, impressive public speaking ability and his seat on the Appropriations Committee, Jackson never faced serious political problems until he became enmeshed in the Blagojevich scandal.

Reid J. Epstein contributed to this report.