CHICAGO — Rep. Jesse Jackson Jr. abruptly, quietly quit Wednesday, effectively ending a once-promising political career months after the civil rights icon’s son went on a mysterious medical leave while facing separate federal investigations.

Just two weeks after voters re-elected him to a ninth full term, Jackson sent his resignation letter to House Speaker John Boehner, citing his ongoing treatment for bipolar disorder and admitting “my share of mistakes.”

The House Ethics Committee is investigating his dealings with imprisoned ex-Illinois Gov. Rod Blagojevich, and for the first time, Jackson publicly acknowledged reports of a new federal probe believed to be looking into his possible misuse of campaign money.

“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 responsibility for my mistakes,” he wrote, adding “they are my mistakes and mine alone.”

Jackson, 47, disappeared in June, and it was later revealed that he was being treated at the Mayo Clinic for bipolar disorder and gastrointestinal issues. He returned to his Washington home in September but went back to the clinic the next month, with his father, the Rev. Jesse Jackson, saying his son had not yet “regained his balance.”

Attempts to locate Jackson Jr. were unsuccessful Wednesday, and family members either declined to comment or could not be reached.

Jackson first took office in 1995 after winning a special election in a largely urban and Democratic district and began his career in Washington with a star power and pedigree that set him apart from his hundreds of other House colleagues.

But despite high expectations, he largely went unnoticed as a policymaker. Instead, he gained a reputation for quixotic pursuits such as trying to impeach President George W. Bush and push through constitutional amendments that had no chance.

He attended an elite private school in Washington and earned a law degree and a master’s in theology. Over the years, he bragged about spending his 21st birthday in jail after being arrested in an anti-apartheid protest, co-authored books with his father and developed his own charismatic speaking style, with near perfect diction and often punctuated by finger pointing.

Shortly after taking office, he was deemed People magazine’s Sexiest Politician in 1997 and became one of the most-outspoken and most-quoted liberals in the House.