But that acrimony did not stop them from throwing back a beer together the day after Election Night at Chicago's legendary Billy Goat tavern, with as much privacy as one could expect from a bar filled by reporters and camera crews.
Kirk suggested that the two men share a beer during his victory speech on Election Night. He said that he and Giannoulias had discussed the possibility earlier in the campaign, and that he wanted to follow through on it. Giannoulias clearly acquiesced.
The Billy Goat's owner, Sam Sianis, set a ground rule: no fighting. After months of doing just that, the former nemeses appeared content to honor the bar-room armistice.
Sobriety seemed more or less to be the order of the proceedings: a sobering defeat Tuesday for Giannoulias, a sobering new responsibility for Kirk, and a sobering agreement ahead of the "beer summit" - a one-beer limit. Kirk drank from a bottle, Giannoulias from a stein.
"We exchanged email and cell phone numbers and I said don't be shy. You know almost half of Illinois voted for him to be their Senator and I think you have to respect that voice," said Kirk. "On camera and thru the TV ads it was tough but off camera he's a very likeable guy."
During the eighteen minute meeting, Giannoulias gave Kirk a book, the Wisdom of Abraham Lincoln.
"I told him, I said, 'look, listen, I know you're going to make some tough votes. Always stay true to yourself.' Who is he to take advice from me but I hope and I believe him when he says he's going to be a good Senator and not worry about the next election," said Giannoulias, donning a Blackhawks baseball cap and jeans.
While it ended with an evening brew with his old opponent, Kirk's first day as senator-elect began just after dawn on the radio and at a train station.
After a night of victory celebration and very little sleep, Kirk made one of his first Wednesday morning stops at WLS Radio, where he talked about his win, as well as the wins of other Republicans, in Tuesday's election.
"The defeat of Democrats in the House is larger than 1994," Kirk said during the radio interview.
After his radio appearance, he rushed down to Ogilvie Transportation Center, where he shook hands with commuters for a third day in a row, this time to thank them.
The spot is symbolic to Kirk because he said it is where he campaigned 10 years ago with his Republican predecessor in the House of Representatives, John Porter.
Kirk won not only the election for the Senate seat in the next Congress but also the special election to complete the term of Senator Roland Burris.
"I look forward to going to work in the Senate pretty soon," said Kirk. "Since we did elect a senator, not just for six years but for the next 60 days to serve in the lame-duck Congress."
Burris was appointed by former governor Rod Blagojevich to complete President Barack Obama's term in the U.S. Senate.
Senator-Rlect Kirk said he is thinking about what his priorities will be upon taking office.
"Number one, I think we should make sure that a huge tax increase doesn't hit the U.S. economy December 31," said Kirk. "That could threaten a double-dip recession."
Before any of that, Kirk hopes to have a beer with his defeated opponent. He's invited Giannoulias to share a drink with him at the Billy Goat Tavern downtown Wednesday night.
"Alexi and I talked about this yesterday, and I think he was amenable. My hope is that Alexi and I can sit down at 5 o'clock today talk about this at the Billy Goat and set another example, which is a pretty tough campaign where two candidates go at it, but then once the people have spoken, you let bygones be bygones and move the country forward," said Kirk.
The Giannoulias camp made no public statements Wednesday morning. Kirk defeated Giannoulias Tuesday 48 percent to 46 percent.
Kirk said that he has already spoken to Senator Dirk Durbin. Kirk says he believes they can work together as long as they can agree on cutting spending and not raising taxes.