The ballot is set for the 6th District, expected to be one of the most closely-watched Congressional races in the country.
Incumbent Republican Rep. Peter Roskam will face challenger Sean Casten who won the Democratic nomination out of a crowded field of seven candidates in the primary election.
The suburban 6th District has been traditionally Republican, but Democrats see it as an opportunity to turn blue after President Donald Trump lost it by 7 points to Hillary Clinton in the 2016 presidential campaign.
Five women ran on the Democratic ballot, but it was scientist and entrepreneur Sean Casten who pulled off a narrow victory and will now face Roskam in the general.
Roskam's votes on tax reform and to repeal the Affordable Care Act, plus criticism over his refusal to hold town hall meetings with constituents in person, were among the issues that spurred so many candidates to try and unseat him.
"I am totally flattered that for the first time in almost 10 years, Roskam would like to come out and meet with people," Casten said after clinching the nomination, already agreeing to a debate with Roskam.
Roskam has been in Congress for 12 years and was already swinging.
"I think it's one thing to talk about things and, kind of like President Trump, tweet about things and so forth, and it's another thing to get things done," he said.
But Casten believes that the numbers are in his favor. The number of 6th District Democratic voters in the 2014 primary, the most recent midterm election year, was more than 8,600, but jumped to nearly 63,000 this year.
"There's so much energy in the Democratic Party around, 'Let's get government back to what government is supposed to do," Casten said.
And how might the bruising fight incumbent GOP Gov. Bruce Rauner is expected to face impact the race?
"I think the low favorability of about everybody in my state has an impact negatively and so look, there's nobody that you really know these days who looks at the political environment and says, 'Oh, I just love it,'" Roskam said, adding, "Now we'll see what the general electorate says."
But Casten says Roskam "has a very hard time defending his legislative record" in a race that has and will continue to garner national attention.
"This is definitely going to be basically ground zero for national Democrats," political strategist Lance Trover said.
"If they can take out the likes of Peter Roskam, they're gonna have a very good night come November," he added. "But I will say this about Peter Roskam: he is a prolific fundraiser with about $2 million in the bank."
However, the other trend that may have Roskam concerned - the number of voters who pulled a Republican ballot on Tuesday was about 10,000 fewer than those who did so in the 2014 primary election.