Civil rights campaigner the Rev Jesse Jackson said his son had quit politics to concentrate on battling mental health problems.
Jesse Jackson Jr's resignation on Wednesday, just two weeks after voters re-elected him to a ninth full term in the US Congress, came months after he went on a mysterious medical leave while facing separate government investigations.
His father told reporters that his son resigned because he did not believe he could continue with his political career and try to regain his health at the same time. "He made the decision to choose his health," he said.
He said that his son had wanted to hold a news conference to discuss his decision to step down, but did not believe he could do so without "breaking down".
He also said there was no way of knowing how long it would take for his son to recover from what he characterised as an "internal unresolved challenge". "It's not the kind of illness you can put a timetable on," Mr Jackson said, adding that he was confident that his son "will get well in time".
Meanwhile, the House Ethics Committee is investigating his son's dealings with imprisoned ex- Illinois governor Rod Blagojevich, and for the first time Mr Jackson Jr publicly acknowledged reports of a new government probe believed to be over 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, co-operate with the investigators, and accept responsibility for my mistakes," he wrote, adding: "They are my mistakes and mine alone."
Mr 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 saying his son had not yet "regained his balance".
Mr Jackson Jr 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 Bush and push through constitutional amendments that had no chance.