Rep. Jesse Jackson, Jr. (D-Ill.) walks up the steps of the U.S. Capitol in this photo from Dec. 2, 2011. Jackson has not been seen in public since June 8. (YURI GRIPAS/REUTERS)

Hours before Barack Obama accepted the Democratic nomination for president at Invesco Field in Denver in 2008, Rep. Jesse L. Jackson Jr. (D-Ill.) settled into the best seat in the house — front row, dead center. Rep. John Lewis (D-Ga.) came along and sat next to him, and the entire arc of the American civil rights movement seemed contained in the moment.

Jackson, the scion of a legendary civil rights leader sitting next to Lewis, an American icon, waiting to watch Obama make history.

The two hugged. “It was a very moving day for him and myself. We didn’t need to speak in a verbal way,” Lewis, now 72, recalled Thursday.

The moment seemed rich with possibility for Jackson. Obama’s triumph promised to open opportunities that were specific to Jackson. An Obama victory in November meant an open Senate seat in Illinois for which Jackson would be a front-runner and an African American president could also change the career trajectory of young, black politicians everywhere.

But those events that were supposed to propel him forward are the ones that seemed to lead to his undoing. What followed for Jackson, 47, has been in a political, professional and personal free fall that has left his career in tatters and his political future in question.

Once regarded as a leading candidate to succeed Obama in the Senate and often mentioned as a possible future mayor of Chicago, Jackson has not been seen in public since June 8.

The explanation for his absence has evolved over the past five weeks. At first, aides said Jackson was experiencing “exhaustion” that resulted in medical treatment. The most recent explanation is that he has a severe “mood disorder” that is likely to keep him away from Capitol Hill until at least September. According to a statement from his office Wednesday, Jackson “is receiving intensive medical treatment at a residential treatment facility for a mood disorder . . . and is expected to make a full recovery.” Aides declined to say where he is being treated.

Jackson’s leave of absence, which his staff says officially began on June 10, came just as federal prosecutors in Chicago charged Raghuveer Nayak, his former fund-raiser, with $1.8 million in medical fraud. Those charges are not formally linked to “Operation Board Games,” a sprawling state corruption probe run by the U.S. attorney’s office that has led to criminal charges against 17 officials, including now-imprisoned former governor Rod Blagojevich (D). Nayak, however, has been in the cross hairs of the federal prosecutors for several years, as he has reportedly testified that Jackson asked him to raise campaign money for the then-governor in 2008 in hopes that Blagojevich would appoint him to Obama’s Senate seat.

For Jackson’s supporters in Washington and Chicago, these developments have been painful. “This is one of the smartest, most capable members of Congress,” Rep. Chaka Fattah (D-Pa.), who serves on the Appropriations Committee with Jackson, said Thursday.

But after a week in which Jackson’s staff has been forced to try to dispel an array of rumors, including one that he is in an Arizona facility being treated for alcohol and drug problems, his friends in the Congressional Black Caucus are whispering that they expect him to resign.

There are two general types of mood disorders: depression and mania. Combined, they cause manic depression, or bipolar disorder, which is an increasingly common diagnosis. There are numerous subtypes of depression, such as dysthymia — a less-severe but longer-lasting form — as well as mood disorders associated with alcohol and drug use.

The most common reason for hospitalization is prevention of suicide or other self-harm, several experts said. “It’s really very difficult to conceal depression,” said Samuel Barondes, director of the Center for Neurobiology and Psychiatry at the University of California at San Francisco. “His handlers may be concerned that he might do things or say things that won’t damage anybody but that will damage his reputation.”

Jackson was one of the early supporters of Obama’s fledgling presidential campaign, even as many older CBC members, including Lewis, sided with Hillary Rodham Clinton. His father, the Rev. Jesse Jackson, publicly expressed doubts about Obama, leading to disagreements between father and son throughout the 2008 campaign.

For the younger Jackson, the Obama campaign wasn’t just about passing the torch to the next crop of black leaders; it was also about Jackson’s ambition. By the summer of 2008, he was among a handful of Illinois politicians courting the Senate appointment as Obama secured the nomination and then took a commanding lead in the general-election campaign.

Court-sanctioned wiretaps would later reveal that Blagojevich was essentially auctioning off the appointment to Obama’s Senate seat to whoever could raise him the most money.

Court documents showed that Nayak discussed raising $1.5 million for the governor, and Jackson met with Blagojevich to discuss the appointment just days before FBI agents arrested the governor. The House ethics committee is investigating the matter.

In late 2010, Jackson’s wife, Chicago Alderman Sandi Jackson, told the Chicago Tribune about an affair the congressman had that nearly ended their marriage. After congressional redistricting pushed his district deeper into the suburbs south of Chicago, Jackson had to fight off a primary challenge, and his career — which once seemed headed for great national ambitions — seems stuck in the House. But his friends are holding out hope that he will be able to rebound soon.

Said Lewis: “He’s always a person of hope and optimism, always looking to the future.”

David Brown, Ed O’Keefe and Alyssa A. Botelho contributed to this report.