(CNN) -- Rep. Jesse Jackson Jr. is suffering from "serious depression -- deep, deep depression," his longtime friend and former U.S. Rep. Patrick Kennedy told CNN Friday, a day after meeting with Jackson at the Mayo Clinic in Rochester, Minnesota.
"Jesse is getting the help he needs, and he needs to make that his priority," Kennedy said about the Illinois Democrat who is undergoing inpatient treatment for bipolar depression at the famed facility.
Jackson, 47, has not been on Capitol Hill since late May. In early June, his office released a statement saying that the congressman, who has served in the U.S. House since 1995, was taking a leave of absence because he was suffering from a "medical condition."
Jackson's office released a statement last month, attributed to an unidentified doctor, that said the congressman was "receiving intensive medical treatment at a residential treatment facility for a mood disorder."
Though exhaustion was initially suspected as the cause of Jackson's treatment, his father, the Rev. Jesse Jackson, later said the issue was "something much deeper, much broader, and it lasted much longer."
"He is suffering from a behavioral symptom, and that is depression, which doesn't allow him to really work to his maximum capacity," Kennedy said.
Kennedy, who himself has been treated for depression, predicted that Jackson will address his constituents within a couple of weeks "about what his intentions are as to whether he's going to continue in public life or whether he is going to continue to focus on his long-term recovery."
Jackson's office has remained open for constituent services.
Noting that mental illness tends to carry a stigma in the United States, Kennedy said he understood why Jackson had not initially addressed the matter in a public forum. "The fact that Jesse didn't talk about this as a mental illness early on is reminiscent of most Americans' experience: if you have a mental illness, you don't talk about it."
In addition to sharing a longtime friendship, the two Democrats both hail from high-profile families and both have suffered mental illness. Kennedy, too, underwent treatment at the Mayo Clinic.
On Monday, the Rochester, Minnesota-based facility said Jackson was responding well to treatment for bipolar II disorder. The mental illness "is a treatable condition that affects parts of the brain controlling emotion, thought and drive and is most likely caused by a complex set of genetic and environmental factors," the Mayo Clinic said in its statement.
Jackson is the son of the famed civil rights leader and represents Illinois' 2nd Congressional District, which includes parts of Chicago. Kennedy formerly represented Rhode Island and is the son of the late Sen. Edward Kennedy, D-Massachusetts.
In addition to struggling openly with addiction, Kennedy has battled bipolar disorder, and has used his public persona to advocate for others with mental illness. He introduced legislation in the House that became law in 2008 forcing insurance companies to cover mental illnesses as if they would physical ailments.
In March, Jackson decisively won a heated primary despite being the subject of an investigation by the House Ethics Committee. The panel has been examining allegations that Jackson or one of his associates offered to raise funds for disgraced former Illinois Gov. Rod Blagojevich in exchange for the U.S. Senate seat vacated by President Barack Obama.
"There is no doubt that the stress in his life, particularly because he's under investigation, clearly perhaps precipitated this," Kennedy said. "Many of us have genetic predispositions to cancer, heart disease or, in this case, mental illness, but they often get triggered by environmental factors. In mental illness, stress is an environmental factor and clearly, I think, has been a factor in his succumbing to this outbreak of his bipolar disorder."
Blagojevich, convicted on multiple corruption charges, started serving a 14-year prison sentence in March.
CNN's Ashleigh Banfield, Laurie Ure and Allison Brennan contributed to this report.