Trotter told police the gun that was confiscated was the same one he uses for work as a security guard, police said.
Trotter has a valid firearms owner identification card and a permit with the Illinois Department of Financial and Professional Regulations to work as a security guard.
Trotter, a Democrat who has served in the state legislature since 1988, will spend Wednesday night in police lockup.
According to police, Trotter was going through security at O'Hare Airport around 7 Wednesday morning when a Transportation Security Administration agent spotted a .25 caliber Beretta in his garment bag via x-ray.
Trotter told investigators he worked late Tuesday night at his job as a security guard at All Points Security, packed his bag early this morning but didnt realize the gun was in his bag, police said.
Last week, Trotter announced he would seek the Congressional seat left open when Jesse Jackson Junior resigned.Trotter is one of more than half a dozen contenders for the open Second Congressional District seat vacated by Jesse Jackson Jr.
Congressman Adam Kinzinger, who used represent part of the district Trotter is seeking to represent, said it could hurt Trotter's chances in the race.
"As far as political, the people of the district and the party leaders have to make a decision to put a person with this kind of baggage in the answer in the Second District," Kinzinger said.
One political analyst says while this arrest might not derail his campaign, it doesn't help in a district that has already seen more than its share of controversial candidates.
"One doesn't understand how one can go on a plane this day and age," the Director of the Institute for Politics at Roosevelt University Paul Green said. "It will be used as an argument against him. It's something he did but not a knock out."
He remains in police custody and will appear in bond court at 26th and California on Thursday at 11:45 a.m.