He never outgrew the diminutives.

As a little boy with an iconic father, he answered to “fella” as he skittered at the feet of civil rights leaders and celebrities. He’s just “Junior” now, even in his mid-40s, a coil of ambition and inscrutability who aspired to be a big thing in politics.

Serving in Congress wasn’t that big thing for Jesse Jackson Jr., a Democrat from Illinois and son of the Rev. Jesse Jackson. Being one of 435 didn’t equate to greatness in the son’s mind. Not back home in Chicago, a place with its own sense of hierarchies, a place where he wanted so desperately to impress.

“Junior is a very insecure person. When you’re in Chicago and you come in and say you’re in Congress . . . Congress is nuthin’. But the mayor of Chicago? He is a boss,” says Frank Coconate, a political operative who helped Jackson test the possibility of a mayoral run several years ago before they had a falling out.

No one talks about Jackson occupying the mayor’s office anymore, and the predictions that he might someday run for president — forecasts that accompanied his arrival in Congress 16 years ago — are long forgotten. Instead, Jackson’s promising career has devolved into a blur of self-destruction, mystery and tabloid drama. He’s been caught in an extramarital affair and been pulled into the pay-for-play scandal that toppled Rod Blagojevich as Illinois governor and prompted a House ethics investigation. He’s disappeared without explanation from Congress, only to surface while receiving treatment for depression and, as first reported by the Chicago Sun-Times and confirmed with law enforcement sources Monday, he is under federal criminal investigation for allegedly misusing campaign funds to decorate his Washington home.

Jackson’s family did not respond to repeated requests for comment, and numerous Jackson allies also declined to be interviewed.

Jackson hasn’t appeared in public since June 8, even though he’s up for reelection next month. On that day, the congressman — who had served as national co-chairman for Barack Obama’s 2008 presidential campaign — announced a plan to increase the minimum wage and accused the president of failing to live up to a campaign promise to lift the wage. It was classic Jesse Jr. — sharp, articulate and utterly quixotic. Few gave him a chance of passing the bill that he announced with Rep. Dennis Kucinich (D-Ohio) and liberal crusader Ralph Nader by his side.

Then Junior was gone.

Shifting stories

Scraps of information surfaced — fragments of the story, a drip-drip-drip that frustrated rather than sated. More than two weeks after his final public appearance, Jackson’s staff said he’d gone on leave June 10 because of “exhaustion.” Capitol Hill staffers were growing frustrated because they couldn’t get any information about Jackson. House Minority Leader Nancy Pelosi (D-Calif.) and Minority Whip Steny H. Hoyer (D-Md.) urged Jackson to reveal more about what was happening. Two weeks after that first announcement, there was another clipped statement — this time a “mood disorder” was cited — and his staff tried to shoot down an NBC report that he was receiving treatment for alcohol abuse.

Another two weeks passed, and now the Mayo Clinic was saying he was being treated for “depression and gastrointestinal issues.” And in mid-August, Mayo announced that Jackson had been diagnosed with bipolar II disorder. The statement noted that he had undergone weight-loss surgery in 2004, a procedure the clinic said could lead to difficulty absorbing minerals and medicines.

“The information we’ve been given has been sparse,” said Rich Hofeld, a staunch Jackson ally who is mayor of the Village of Homewood, a Chicago suburb. “I think people kind of raise their eyebrows and question what is going on.”

Jackson is recuperating at his home in Washington after spending at least six weeks at the Mayo Clinic, where he was visited by Kucinich and former congressman Patrick Kennedy. His staff hasn’t said when he will return. Few colleagues have seen him, but Rep. Emanuel Cleaver II (D-Mo.) said in a brief interview that he dined with Jackson recently and has spent time with Rev. Jackson. “They’re trying to make sure that he is completely well before he comes back,” Cleaver said. A Cleaver spokesman later said the congressman had met with Jackson over the course of a few weeks and that their conversations have been “solely of a pastoral nature.”

Jackson’s mother, Jacqueline Jackson, grasped for an explanation during surprise remarks about her son at a Rainbow/PUSH gathering: “He thought he was going to be the senator — thought he was going to have a chance to run for mayor,” she said. “Young people don’t bounce back with disappointments like me and my husband.”

Reporters were ambushing the son’s wife, Chicago Alderman Sandi Jackson, at city council meetings, pressing her about speculation that her husband would drop out of his congressional race and that she would step in to replace him. She showed the strain by calling them “vultures.”

The cryptic and confusing sequence of announcements about Jackson’s health reveals something about the delicate position Jackson finds himself in and about the nature of Jackson’s world, a place with a center of gravity in Chicago, rather than Washington.

Jackson’s staff was preparing to announce his leave of absence on June 18, according to a person familiar with the planning. But the statement had to be cleared by both his father and his wife, and they were moving slowly.

By the time they’d signed off, another crisis hit: Jackson’s friend Raghuveer Nayak was arrested in Chicago and charged with paying kickbacks to doctors in exchange for referrals to his surgery centers. Nayak was linked to the Blagojevich case because he allegedly offered to raise $1 million for the governor if he named Jackson to the Senate seat vacated when Obama was elected president. Nayak had also allegedly paid to fly a woman suspected of having an affair with Jackson to Chicago.

Jackson’s staffers were concerned that a media narrative would develop: He had taken the leave because of distress about the Nayak investigation. So they waited. It wasn’t until June 25 that they announced he had taken a leave because of exhaustion. They averted one media narrative and created another: They had been holding back information.

In fact, Jackson’s staffers knew almost nothing about what was happening, a congressional source said, and they weren’t happy about being placed in that position.

A focus on Chicago

Jackson’s focus has always seemed to be in Chicago, rather than in Washington. He displayed a different persona in each place, former associates say. In Washington, “he never became a player,” a top Democratic aide said. He struck colleagues as erratic. They’d see him late in the evening, walking the hall in martial arts gear, but miss him at meetings. On karaoke night at the Democratic Club, he could wow friends with his deft dance moves, but at other times he would retreat from social contact, according to interviews with several top Democratic aides.

“He’s not a creature of the Hill,” one top aide said. “Jesse came with a lot of fanfare because of his name. That brought a certain level of expectation. I think there’s a sentiment that he’s never lived up to it.”

But if he was perceived as irrelevant in Washington, he had real swagger in Chicago politics. “He fights all his fights on the local level,” said Debbie Halvorson, a Democrat who served with Jackson in the House before losing a reelection bid in 2010. Halvorson was soundly defeated by Jackson in a Democratic congressional primary in March.

“He is very talented and he was building his own machine; he was like the prince on the South Side,” Coconate, the political operative, recalled.

Building a power base

The prince was erecting the foundations of a power base, diving into countless local races by endorsing candidates, seeding the region with mayors and other local officials who would be loyal supporters. But he could be mercurial, former associates say. Jackson sometimes boasted that he was a reincarnated Greek chariot driver, Coconate said. “I really thought he had a problem with reality,” Coconate said. “He’d get in his own little world. He’d come out with outlandish things.” At one of Jackson’s hangouts — a Turkish bath — he’d prance naked, demonstrating martial arts moves, while the others stayed wrapped in towels, said Frank Avila Jr., a former supporter who is a Democratic operative.

Jackson’s language could be shockingly inappropriate, especially for someone who grew up at the epicenter of the modern civil rights movement, former associates say. Coconate, who is of Italian heritage, recalls Jackson calling him “Scungilli Head,” a reference to an Italian seafood dish. Jackson called older African American lawmakers “plantation Negroes” and whites were “lemonheads,” according to Coconate and Avila. Frank Watkins, a spokesman for Jackson’s congressional office said, “I’ve never heard him say anything remotely like that. It sounds completely foreign to me.”

Avila was doubtful about Jackson’s offer to help him in what would eventually be an unsuccessful race for Water Reclamation District commissioner. It was a minor race, he said, but Jackson insisted that he take a $12,000 poll, using Jackson’s pollster. Jackson also required him to use an expensive photographer to snap shots of them together, only to discover that he would merely be digitally added to a stock image of Jackson.

Campaign spending questions have been raised for years about Jackson. In 2007 and 2008, Jackson’s campaign spent more than $22,000 on nine trips to Ozio, a restaurant on M Street in downtown Washington, where his alleged mistress — a former swimsuit model — worked.

Jackson’s current campaign is a baffling abstraction, but he’s expected to win easily against a Republican challenger in his heavily Democratic district. His wife and campaign spokesman say he is on the ballot and plans to serve if elected. But no events are scheduled, and Jackson’s campaign spokesman, Kevin Lampe, declined to say whether any advertisements or mailers have been purchased.

Speculation that Jackson might drop out of the race, before or after the election, is rampant, stoked in part by his decision to put his home in the Dupont Circle area up for sale. Sandi Jackson has said the sale was prompted because of a need to pay for her husband’s medical expenses. The home was taken off the market after a flurry of reports about the sale, but Lampe says it’s still available as “a private listing.”

“The unknowing leaves the mind to wander,” said Illinois state Sen. Emil Jones III, whose father, the South Side power broker Emil Jones Jr., was defeated in Jackson’s first run for Congress in 1995. Jones has heard the theory that Jackson has felt the strain of living up to his famous father’s legacy, but he doesn’t think much of it.

“There’s still a lot of unanswered questions with what happened with him and Blagojevich,” Jones Jr. said. “I believe that has a lot more to do with it than the pressures of being the son of a great leader.”

While he chatted in his father’s South Side restaurant, Jones’s cellphone rang. He took the call. It was a local politician with an eye on a congressional seat — Junior’s.

Alice Crites, Sari Horwitz, Paul Kane and Peter Wallsten contributed to this report.