Democratic congressional candidate Brad Harriman has dropped out of the 12th District race against Republican candidate Jason Plummer, citing a neurological illness that he said requires surgery.

Harriman said he made the decision after a discussion with his doctor.

"Today, it is with a heavy heart that I must announce that I am ending my campaign after consultation with my doctor. I know in my heart that this decision is in the best interest of the voters of Southern Illinois who deserve a candidate that can withstand the pace that this race will require," Harriman said.

He added, "My condition has noticeably worsened over the course of the campaign to the point that if I do not address it with surgery, I am facing irreparable damage. While it is nonlife-threatening, I need to address it now."

Speculation quickly filled political circles on who would replace Harriman on the ballot. One potential candidate, state Rep. Jerry Costello II, D-Smithon, said Tuesday he has no interest in running for Congress, while another, state Rep. John Bradley, D-Marion, called the prospect intriguing.

Harriman, who previously served as St. Clair County regional superintendent of schools, said he has lived with a neurological condition without limitation or noticeable change since 2010, though he began noticing worsening symptoms in May and underwent testing to determine whether his condition had worsened. His physician advised of long-term and permanent injury if the condition is left untreated and allowed to worsen over the course of the campaign.

U.S. Rep. Jerry Costello, D-Belleville, who has held the 12th District seat for the past 22 years, said Tuesday he won't reconsider his decision to step aside and not seek re-election.

"Brad's decision to end his campaign due to health problems is understandable, and I wish Brad the very best in the future," Congessman Costello said. "As you know, I announced last October that I will not seek re-election, and I am not going to reconsider and will not be a candidate for re-election."

The congressman said the search for a replacement candidate will begin soon.

"The law is very clear on the procedure to select a successor. The Democratic Party chairs from each of the 12 counties in the congressional district will meet to decide on a candidate to fill the vacancy," the congressman said. "Each chairman will have a weighted vote based on the number of Democratic votes cast in the March primary election in their respective county," he said.

Costello said he would co-chair the selection committee to find another candidate and that those discussions would begin soon.

"Under the law, as the elected Democratic state central committeewoman and committeeman, Barb Brown and I will co-chair the selection committee and will recommend that they follow an open process to select the most qualified candidate. Committeewoman Brown and I will discuss the process with the 12 county chairs and make a public announcement soon," Costello said.

Congressman Costello was in Springfield on Tuesday, visiting with state lawmakers. He made a visit to the office of Illinois House Speaker Michael Madigan, D-Chicago.

In a statement, Plummer said, "My thoughts and prayers go out to Brad and his family as he deals with his medical condition. He had a distinguished career as an educator and leader in the area, and I wish him the best.

"While I enjoyed the opportunity to get to know him on the campaign trail, this race was never about Brad or myself. This race is about and will be about Southern Illinoisians electing a congressman who will represent their values and fight the ballooning federal government.

"My campaign will continue to provide residents of the 12th District a new direction to shrink the size of government, reduce regulations on small businesses and best utilize the natural resources of our state," Plummer said.

St. Clair County Republican Party Chairman Jon McLean said he had an inkling that something was wrong on the Democrat's side.

"I never thought they were serious with Harriman to begin with, since they had to go through so many candidates to find someone to run. And then he's been nowhere during the campaign, he hasn't been at any events in the district," McLean said.

McLean said Democrats will have to quickly find a replacement who will be able to compete in what promises to be a rigorous campaign. He said a Democrat in Southern Illinois will have an uphill battle because the residents "just cannot relate to Obama."

Harriman said he knew it would be a long, hard campaign.

"I know the rigors that this campaign will demand, and I know that my health will prevent me from running the kind of campaign that Southern Illinoisans should expect. Over the course of the past seven months of this campaign, I have been humbled by the support I have received from hardworking Southern Illinoisans, and I will always be grateful to those that invited me into their homes and their businesses," Harriman said. "For those individuals and the future of southern Illinois, I have confidence that the democratic county chairmen will work together in the most transparent way possible to select a candidate that will continue to fight for the Southern Illinois way of life."

The race will decide the successor to Congressman Costello, in a district that covers southwest Illinois, from the Granite City area to Cairo. Costello is retiring. The race is being eyed by the national parties and is expected to cost millions of dollars.

McLean said he expects that Jerry Costello II will be considered as a potential candidate. But the younger Costello is currently serving a term in the Illinois House and is planning to run for the state House seat this fall.

"I've made a commitment to run in the 116th (state House) district for state representative. I'm absolutely going to run for re-election in the 116th," Costello II said.

McLean also said he has heard that state Rep. John Bradley, D-Marion, is being considered to run. When asked about the prospect, Bradley said, "It's intriguing," and declined further comment.

Madison County Chief Judge Ann Callis had been mentioned previously as a possible candidate, and voiced interest in the race early on, but she later chose not to run. She was not immediately available for comment Tuesday.

St. Clair County Board Chairman Mark Kern said he is not a candidate to step in for Harriman.

"I am in the middle of trying to be re-elected St. Clair County Board chairman," Kern said. "It's going to be an open process. I'm certain that we'll find a good candidate."

Kern didn't offer any predictions about who that candidate might be.

St. Clair County Democratic Party Chairman Robert Sprague was not immediately available for comment.

Harriman's exodus caught some political observers off-guard.

"It's a total surprise -- a big but not fatal setback to the Democrats" just five months before the election, said David Yepsen, director of the Paul Simon Public Policy Institute at Southern Illinois University in Carbondale.

Yepsen said Tuesday's development is at least a short-term setback for Democrats, noting any Harriman replacement would have to play campaign catch-up with Plummer. But "if they can find a good replacement candidate, I expect that by the fall the race can be competitive again," Yepsen said.

"The Democrats do have time to get someone back in the game," he said.

The 12th District could be the one spot where Republicans wrest away a seat in a year when Illinois Democrats hope a redrawn political map allows them to undo GOP gains in 2010 as part of efforts to win back a majority in the House.

Congressional analysts say it is a critical seat for Democrats to hold, but Republicans are banking on the area's conservative leanings, angst over environmental regulations relating to coal and signs of frustration with Washington and Illinois' Democratic leadership.

"All of us expected this to be a close race, one of the real battlegrounds in the race for control of the U.S. House," Yepsen said.

While Democrats say they are confident of maintaining the seat, the GOP takes heart from 2010 election results in which other parts of downstate Illinois shifted its way.