The first debate between those two candidates played out in front of a national audience on NBC's Meet the Press.
Kirk and Giannoulias took aim at each other's parties and each other, with Giannoulias saying Kirk had told some "whoppers" during the campaign.
The candidates sat side by side after months of attacks on each other from afar. It was their first debate Sunday during which, predictably, Republican candidate Kirk criticized the Obama administration's handling of the economy, and Democrat Giannoulias defended the administration for preventing a second Great Depression.
When pressed, the candidates addressed nagging issues: for Giannoulias, it was his involvement in his family's failed bank. Kirk held up a list of felons who allegedly got financing through Broadway Bank.
"They gave millions of dollars after the felons had been convicted," Kirk said.
"For the congressman to characterize my family that way is misleading. It's offensive. People aren't buying it, and the congressman wouldn't know the difference because he has been in D.C. for 20 years," Giannoulias said.
Another lingering issue was Kirk's military record. Sunday morning, he was called on for mistaking his military service.
"I made mistakes with regard to my military misstatements. I was careless, and I learned a very painful and humbling lesson," said Kirk.
"What's more troubling than the untruths about his military record, than his phantom teaching, to me are his votes in Washington, D.C.," Giannoulias said.
Sunday's debate did not include all of the candidates in the race. The Green Party candidate and Libertarian candidate spoke with ABC7 Chicago after the debate between Kirk and Giannoulias.
"We are on the ballot, and we have just as much concern about issues as those candidates, and we were not afforded the opportunity to put forth those positions before a national audience," Green Party candidate Lealan Jones said.
"We sat together and watched the debate, and then we had a 45-minute debate amongst ourselves, and all we did was talk about issues," said Mike Labno, the Libertarian candidate in the race.
Kirk and Giannoulias are scheduled to debate three more times in October.