US Representative Jesse Jackson Jr, who has been treated for bipolar disorder and is reportedly under investigation for alleged misuse of campaign funds, has resigned his seat.
The 47-year-old son of civil rights leader and presidential candidate Reverend Jesse Jackson, also admitted to "my share of mistakes."
He resigned in a two-page letter to John Boehner, the current Speaker of the House of Representatives.
According to news reports citing unnamed sources, Mr Jackson is also being investigated by the FBI over possible misuse of campaign money.
In his letter to Mr Boehner, the Democratic Party member acknowledged a probe was underway, and said he hoped it would not overshadow his years of public service.
Mr Jackson has been in Congress since 1995 and easily won re-election in a heavily Democratic district earlier this month despite his ailment and the ethics questions hanging over him.
His resignation is not expected to result in any changes in the political balance of power of Congress, which is controlled by Republicans.
Mr Jackson was treated for at least six weeks at the Mayo Clinic in Rochester, Minnesota, this summer for bipolar disorder, a psychological condition marked by extreme mood swings, and has been on medical leave from the House since June.
In addition to his health issues, Mr Jackson has been the subject of a House ethics committee probe over an alleged bribe offered by a Jackson supporter in 2008 to then Illinois Governor Rod Blagojevich.
The bribe was said to be intended to entice Blagojevich to appoint Mr Jackson to the US Senate seat vacated by President Barack Obama.
Mr Jackson has admitted to lobbying for the seat, but denied knowing about any money offered to Blagojevich, who has since been convicted on corruption charges and imprisoned.