05/30/2012 04:35 pm ET Updated May 30, 2012

Illinois 12th Race Loses Democratic Candidate, Brad Harriman, Due To Illness

Citing a neurological illness, Democratic congressional candidate Brad Harriman suddenly dropped out of the contest for the open seat in Illinois' 12th District on Tuesday.

His withdrawal leaves the Republican nominee, Jason Plummer, alone in the race for the seat being vacated by Rep. Jerry Costello (D-Ill.). Even before Harriman's departure, Republicans were hoping conservative leanings in southwestern Illinois would allow them to wrest the seat away from Democrats, despite new district lines that favor the incumbent party.

Democrats, on the other hand, believe Illinois is the key to regaining control of the U.S. House of Representatives, and they will need to hold the 12th District.

Harriman told reporters his decision came after a discussion with his doctor, adding that "the voters of southern Illinois ... deserve a candidate that can withstand the pace that this race will require."

Harriman said his illness, which he has had since 2010 and which he did not specifically name, began to worsen noticeably in May. After testing, his physician warned him of the possibility of sustained and permanent injury if Harriman left the condition untreated for the duration of his campaign.

Plummer said his thoughts and prayers were with his one-time rival and the Harriman family.

"While I enjoyed the opportunity to get to know him on the campaign trail, this race was never about Brad or myself," Plummer noted in a statement. He added, "This race is about and will be about southern Illinoisans electing a congressman who will represent their values."

Plummer, who ran unsuccessfully for lieutenant governor in 2010, was criticized for refusing to release his tax returns in that race. He was also mocked by Jay Leno for several awkward television appearances.

A committee of local Democratic Party chairs, led by Rep. Costello, will choose a replacement nominee in the next few weeks. Their votes will be weighted based on the number of votes cast in each county in the March primary. The process will mirror that used by local GOP chairs in Illinois' 13th District, when they selected Rodney Davis to replace Rep. Tim Johnson on the ballot after Johnson's abrupt retirement.

Costello will not reverse his decision to retire, the Belleville News Democrat reports.

"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," Costello said.

The incumbent's son, Jerry Costello II, a Democrat who serves in the Illinois House of Representatives, was being discussed as a potential candidate, but he said on Tuesday he has no interest in the race.