- He defeated Democratic State Treasurer Alexi Giannoulias by a slim margin of 1.6 percentage points; 59,220 votes among 3,704, 473 votes cast.
That's a long way of saying that Kirk didn't exactly ascend to the U.S. Senate on a wave of popular support in 2010. He was helped considerably by questions, fair or unfair, about his opponent's business dealings and various other factors.
Which is why Kirk, perhaps more than any Republican candidate in the nation, has reason for concern if Donald Trump becomes the Republican presidential candidate. Historically, Democratic voter turnout is higher in Illinois in presidential election years.
Given Trump's polarizing effect -- not to mention the genuine fear he instills in many voters -- his presence atop the ballot on Election Day will provide even more incentive for Democrats to get to the polls.
No one is more acutely aware of this than Kirk's Democratic opponent, U.S. Rep. Tammy Duckworth, whose campaign on Monday launched a blistering effort to tie Kirk to Trump in as unforgiving a fashion as possible.
In less than a minute, the video links Kirk's statement that he'd support Trump if Trump is the Republican nominee with many of the more ridiculous statements Trump has made during the campaign. Mexican rapists, John McCain not being a war hero because he was captured, comments about Fox's Megyn Kelly bleeding "from wherever," Obama being a foreign-born Muslim... all those statements are there, interspersed repeatedly with the same NBC5 Chicago interview in which Kirk says that he'd support Trump as the GOP nominee.
The video ends with the kicker title, "Kirk and Trump: Making America great?"
The video no doubt will galvanize the 557,000 Republican voters who helped Trump earn a solid victory in the March 15 primary. In all, 1.4 million Illinois Republicans cast ballots in the primary.
But Hillary Clinton received 1.02 million votes on the Democratic side, where the vote total was 2.02 million. There were a whole lot more Democratic voters out for the primary, and Kirk will need to win a good number of them -- as he did in 2010 -- to defeat Duckworth.
And if the prospect of beating back a Donald Trump presidency isn't incentive enough to give Democratic voters extra motivation, a new election law passed in the final weeks of Gov. Pat Quinn's administration will expand early voting and same-day voter registration for the November election. The changes are thought to benefit Democrats in getting out the vote.
The video kicks off a campaign called "112 Days of Trump," in which the Duckworth camp plans to count down the remaining days until the Republican National Convention in Cleveland "to highlight how the embarrassing duo of Trump & Kirk won't make America great again," says a press release.
The Washington Post this month reiterated its opinion that the Kirk re-election race was the No. 1 race to watch among all Senate incumbents. With the launch of his opponent's effort to portray him as just short of Trump's running mate, this race just got even more interesting -- and perhaps more challenging for the incumbent.