CHICAGO (CBS) – Many political leaders in Chicago were saying it’s a sad day to learn U.S. Rep. Jesse Jackson Jr. has resigned from Congress.

Jackson submitted his resignation letter to House Speaker John Boehner this afternoon, after more than five months of being on medical leave for bipolar disorder. He’s also facing a federal investigation into his campaign finance activities, and his attorneys have been negotiating a plea deal.

Two of Jackson’s closest allies in Congress said it’s a sad day for Chicago that Jackson is stepping down.

“We think that it’s an unfortunate and sad day, not only for the citizens of the 2nd District in Illinois, not only for the residents of Chicago and the surrounding communities, not only for the Jackson family, but we think it’s a sad day for America,” Congressman Danny Davis said.

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Davis said he had hoped Jackson would have been able to recover enough to return to work, but that obviously won’t happen now.

“I think, as a result of the resignation of Representative Jesse Jackson Jr., that America is losing, out of the halls of Congress, one of its most able, most articulate, and most clear voices,” Davis said.

He praised Jackson’s 17 years representing the 2nd Congressional District, and his years of work to bring a third major Chicago area airport to the southern suburbs.

Davis and Congressman Bobby Rush go way back with the Jackson family.

Rush said he remembers when Jackson was a toddler in the 1960s, when rush was a member of the Black Panther Party, and worked with Jackson’s father, Rev. Jesse Jackson Sr.

Rush said he talked with Jackson Jr. today, and it was clear how much he was struggling with mental illness.

“He sounded very, very sorrowful. He sounded … in so much pain,” Rush said. “He was real sincere in his thought that he would not be able to serve in the Congress anymore. I mean, it was a very moving moment.”

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Mayor Rahm Emanuel, who was once a colleague of Jackson’s in Congress, said in a written statement, “While I’m sure this was a difficult decision for Congressman Jackson, now is the time to look forward. The residents of the 2nd Congressional District will now have an opportunity to choose their next leader to fight for all of us in Washington, D.C. My thoughts and prayers and with the Jackson family and I wish Jesse a healthy recovery.”

Congressman Mike Quigley, who replaced Emanuel in Congress, said he’s known Jackson for about 12 years. He said he isn’t surprised to learn Jackson has decided to step down, given the combined pressures of his health issues and the federal investigation.

Quigley said he hasn’t heard anything from Jackson since he began his leave of absence from Congress in June, but said Jackson was a responsive Congressman when he was on the job.

“When Congressman Jackson was there, he rarely missed a vote. He was very conscientious about such things. He cared a great deal about his constituents, and my reaction was that he liked being a congressman,” Quigley said.

He said Jackson’s resignation reminds him that people in elected office need to be accountable, and serve in the most transparent way possible.

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Gov. Pat Quinn, who is now tasked with scheduling a special election to replace Jackson, said it’s sad when someone has to resign from political office, and he wishes the former congressman well.

But his thoughts were also with the people of Illinois.

“Anytime there’s a vacancy in an elected office – whether it’s U.S. Senate or Congressman – we want to make sure Illinois is fully represented, and we have our voice heard in Washington, especially now with all these major decisions that will be occurring in the coming year,” Quinn said.

Quinn, Davis and Rush began careers in public life long before Jackson Jr. was first elected to Congress in 1995. They saw the birth of a promising political career, and now they’re witnessing its demise.