State Sen. Donne Trotter was charged with a felony after TSA agents found a loaded gun in his bag as he went through security at O’Hare International Airport on Dec. 5, 2012. (Credit: Chicago Police)

Updated 12/5/12 – 8:51 p.m.

CHICAGO (CBS) – An Illinois state senator running to replace Jesse Jackson Jr. in Congress was arrested Wednesday morning after he allegedly tried to bring a gun onto a plane at O’Hare International Airport.

Sen. Donne Trotter (D-Chicago) was charged with a felony count of attempting to board an aircraft with a weapon, according to the Cook County State’s Attorney’s office. The charge carries a penalty of 1 to 3 years in jail.

WBBM Newsradio’s Steve Miller reports Chicago police said TSA agents found the gun in a carry-on bag around 7 a.m. as Trotter was going through a security checkpoint. A TSA spokesperson said in an email that the gun was loaded with seven rounds, but there was not a round in the gun’s chamber.

A police spokeswoman said Trotter is licensed to carry a weapon and has a valid Firearm Owner’s Identification card.

LISTEN: WBBM Newsradio’s Steve Miller reports

Trotter was being held at the Jefferson Park District lockup, police said. He was due to appear for a bond hearing Thursday, according to prosecutors.

Police Supt. Garry McCarthy said, “I don’t know why he would have a gun. If it’s a legal gun, he should know better than bringing it on a plane.”

CBS 2′s Mike Parker reports Trotter told authorities he had the gun because he moonlights at Allpoints Security and Detective Inc., and forgot he had the gun in his garment bag when he went to O’Hare for a flight to Washington, D.C.

Trotter’s wife Rose told Newsradio she has no comment.

“I don’t have any more information to add to what you already have,” she said.

An aide to Trotter, Gil Medina, said he’s still trying to reach Trotter.

Employees at the security agency would not comment on the arrest.

Trotter, 62, has been a state senator since 1993. Before that, he was a state representative from 1988 to 1993. During his time in the legislature, he has regularly been an anti-gun vote, and has opposed legislation that would allow for concealed carry in Illinois.

He represents the 17th District of the Illinois Senate, which covers portions the Far South Side and southern suburbs, including the Pullman, Hegewisch and Chatham neighborhoods, and as far south as Kankakee County.

He announced last week that he would run in the special election in the 2nd Congressional District to replace Jesse Jackson Jr., who stepped down last month.

South Side resident Kevin Atchison, who voted for Trotter in the recent election, said Trotter’s arrest could hurt him if he continues to try to run for Jackson’s seat in Congress.

As for Trotter’s explanation that he forgot the gun was in his bag, Atchison said, “Well, that goes around a lot. A lot of people say they forget theirs. So, I’m just totally shocked.”

Trotter also ran for Congress in 2000, against incumbent Congressman Bobby Rush and future President Barack Obama, but came in third in the Democratic primary.

The Illinois Senate was in session on Wednesday. State Sen. John Millner (R-Carol Stream) said, from what little he has heard about the arrest, Trotter’s explanation sounds plausible.

“My understanding of it, when I just heard about this story, is that it was an accident and he that he was employed by some security firm, and that could happen. If you’re in a hurry, and you’re getting off-duty, and you throw your gun in your case, you don’t think about it. That can happen,” Millner said.

He said Trotter has a reputation among his colleagues as “a very professional, hard-working, dedicated public servant,” and that the arrest doesn’t make sense to him, unless it’s the result of an honest mistake by Trotter.

Millner, a former Elmhust police chief, acknowledged that – mistake or not, state senator or not – there’s no provision for airport security to simply wave Trotter through after finding a gun in his bag, unless he has authorization to take a gun on a plane.

“If you carry a weapon, you have to notify them that you have a weapon, you have to have approval to have that weapon,” he said. “You actually are given a form that you have to fill out. It cannot be carried onto the aircraft with you, unless you are a federal law enforcement officer, or other specialized officer that’s allowed to carry while you’re on an aircraft, which is rare.”