Kirk defeated Illinois Treasurer Alexi Giannoulias in one of the nation's ugliest and most closely watched Senate races.
Giannoulias faced attacks over his family's failed bank, which gave loans to two men involved in organized crime. Meanwhile, Kirk was forced to apologize after the disclosure that he had made false claims about his military record.
The Illinois race got more attention than most Senate campaigns because this was the seat held by Barack Obama before his move to the White House.
"We are 800 miles from any ocean, but a tsunami just hit the heartland. My name is Mark Kirk and I'm going to replace Roland Burris in the United States Senate," Kirk proclaimed in his victory speech. "It was a vote for fiscal responsibility, for spending restraints, for lower taxes, for bringing down the deficit, a vote to put our economy back to work, our vote to support the troops, a vote to defend our allies, and in the tradition of this congressional district, a vote for thoughtful, independent leadership."
Kirk said the seat he will occupy has seen dark days.
"Tonight, the sun set on a one-party corrupt state. And this Senate seat was just returned to its rightful owners, the people of Illinois," Kirk said. "The people didn't just elect a senator for the next six years, we also elected a senator for the next 60 days as well. And as your new senator, I pledge to oppose any nonessential, nonemergency spending legislation coming to the lame duck session of the Congress. Tonight, we have heard the American people deliver their verdict on new taxes and spending by the Congress. That mandate is gone."
Kirk thanked Giannoulias for a gracious concession phone call and praised his opponent's mother for doing a good job raising her son.
"Alexi and I, during this campaign, we discussed having a beer when this is all over. And so I will give this invitation. Alexi, if you still want that beer, I'll see you tomorrow night at Lower Wacker Drive [at] Billy Goat Tavern and the first round is on me," Kirk said.
Giannoulias congratulated Jan Schakowsky on her victory and thanked Sen. Dick Durbin for his campaigning in an emotional concession speech.
"I can never repay what he did for me and all the fight and struggle that he put into this campaign. Senator, while I'm not going to be serving with you in the United States Senate, know my respect has only quadrupled, and I'll always be there for you as a friend," Giannoulias said.
Giannoulias also alluded to a possible future in politics.
"Apparently the next campaign is being planned already," he said. "Thank you to my team for doing everything you've done for me. I love you guys as if you were my own family and I'm sorry if I let you down. And we're going to get them. Let's never forget what we ran for."
The new junior senator from Illinois, five-term Congressman Kirk got a vote of confidence in Giannoulias' concession speech, with Giannoulias saying that Kirk will make a good, strong senator.
"Losing is not something you expect, which is probably why I didn't write a speech," Giannoulias said. "But my parents always taught me if something happens and you lose, you always maintain your dignity. And the reason that this is easier than it should be is because I accepted a hundred times, this race was never about me. This race was about something bigger. The fact all of you went to war and worked so hard for almost two years says more about all of you than it does about me, and it shows how much all of you care deeply about this great state and this great country. And I don't want any of you to lose that. I don't want any of you to forget why we did what we did, why we put this campaign together. I got those values from my parents."
Giannoulias thanked his family, alluding to the difficult year of scrutiny his family and their failed bank have faced.
"My two brothers, George and Dmitri, who have had a very difficult year, a very unfair year, but have never lost their dignity, never lost their class, have always been fighters, have always held their head up high. And I am so incredibly proud of my brothers. I'm so incredibly proud of how they love their families, and if it weren't for them, the lessons they taught me, I would never be a candidate for the United States Senate, I wouldn't be a Democratic nominee for the U.S. Senate and wouldn't be the man that I am," Giannoulias said.
Giannoulias encouraged his supporters to stay active in the community and continue to serve others in the spirit of his campaign.
"Every single day we should remember what it means to be a public servant, not just running for office, it's helping someone down the street or doing something nice for someone, it's getting involved in a campaign. So while tonight we didn't get the results that we had hoped for, that we wanted, I think everyone in this room, all my supporters, everyone on this team, everyone on this stage should hold their head up high because we never forgot, not one minute in this campaign what this was all about and we never will," said Giannoulias.
Not only did Kirk win a full tern in the Senate, but he also won the 60-day term between Election Day and the full Senate swearing-in in January. Kirk said he's going to seek a swearing-in as soon as possible for the two months remaining in the Obama term.
"This is a mandate to cut spending and make sure we don't increase the tax burden on the American people. But the Senate is a place that rewards people who play well with others. My job is not to go to the Senate, which is nearly equally divided, and move a pro-Illinois agenda. That means cajoling and making sure that people come to our way. But in the end, it's clear we need spending restraint. We shouldn't have a big tax increase on the by December 31. Now that the people have spoken so clearly, my hope is there's a growing consensus on behalf of a more fiscally conservative policy," Kirk said.
Kirk said he fears a huge last-minute tax increase that will send the country into a deeper recession.
Kirk said his victory has some reflection on the White House but even more reflection on House Speaker Nancy Pelosi and Senate Majority Leader Harry Reid.
"My job is to work as effectively as possible to bring the Senate back to center right and especially with a laser focus on good economics that will grow employment," Kirk said.
The wait for results
There was a sizable and growing crowd at Kirk's campaign headquarters in Wheeling where the staffers were dispensing optimism throughout the night. They set a benchmark for 20 percent support in the city of Chicago, and they say they believe they surpassed it.
As the downstate vote, the DuPage County vote, and collar county votes were still coming in, Republicans salivated at the prospect of taking the Obama Senate seat.
"I think that there's been a lot of discussion about dissatisfaction with the direction of the country and of the state under total Democrat control at both levels, so I think that many people in Illinois are interested in seeing a change. And certainly the opportunity to vote for a Republican U.S. senator and a Republican governor would put a change in direction as to where we've been going," said State Sen. Christine Radogno, (R), Ill. Senate minority leader.
The five-term congressman, after voting Tuesday morning, visited some of his call centers and was waiting in his war room to break down the numbers. While they prepared for the possibility of a long night, some Kirk supporters say this race would not be close if it were not for concerns over resume embellishment by the congressman.
"I think he is got so much integrity and he is as good as good comes. His word is sound. He saved the VA hospital. To me, that's a huge deal," said Carrie Hoza, millitary veteran.
Kirk's late surge in votes is not good news for Giannoulias.
There was cautious optimism and a lot of energy in the Imperial Ballroom in the second floor of the Fairmont Hotel where Giannoulias' camp was set up. The room was filled with Giannoulias supporters hoping for a victory celebration when the night was over.
The former state treasurer was crunching the numbers and watching returns in his hotel suite at the Fairmont as his advisers were going over the votes.
Senior senator Dick Durbin said that while they are cautiously optimistic, no matter who wins, both parties must work together or it will be the American citizens who will suffer the most.
"It's going to be very difficult, challenging. We'll have fewer votes in the United States Senate on the Democratic side. We will need more Republicans to cross over and help us. I heard some of the tea party candidates say we're not compromising on anything. That's not a good environment to work out solutions to the problem," Durbin said.
Final push Tuesday
"You look at the public policy difference, there is no race in the country where there is a starker difference between Kirk and someone who wants to fight for middle class and working families who just want a shot at the American Dream," said Giannoulias.
"We need a check and balance in the United States Senate. Not someone to take us to the extreme right, but how about back to the center, especially focused on cutting spending?" said Kirk.
Giannoulias voted near his North Side home. Kirk voted in his hometown of Highwood. Both candidates think the race will be close and come down to just a few thousand votes.
"We knew it would be tight a long time ago, but this is about turnout," said Giannoulias.
The race has been nasty at times with the candidates sparring in debates and airing negative ads.
"I tell you, if there's ever an election where the candidate runs positive ads, that's who I'd vote for. The mudslinging gets old after awhile," said Tim Dobry, voter.
"I see a lot of unfulfilled promises politicians make, and I think what's most important to me is voting on someone I believe in rather than this stance on an issue where I'm unsure they'll come through on that," said Glen Lorentz, voter.
Giannoulias is counting on a big city turnout, particularly in African-American precincts. Kirk hopes to tap into the reservoir of voter discontent.
"I would say our traffic is as high or higher than last presidential election and significantly higher than any mid-term and I've been doing this for ten years," said Pat Kosloski, election judge at South Park Recreation Center in Park Ridge. That precinct is traditionally Republican, so that may bode well for Kirk in a race where many have wrestled with their decisions.
"We balanced it by his patriotism," said Dorothy Kuenstle, Kirk supporter.
"Kirk is for the rich and Giannoulias is for the poor, and that's why I love him," said Kay Giglio about Giannoulias.
The Associated Press contributed to this report.