If Democrats want to retake the U.S. Senate, they'll need Illinois to do it, according to Matt Streb, a political scientist at Northern Illinois University.
The margin is five seats, or four if Hillary Clinton becomes president; that would allow her vice president to cast tie-breaking votes. Right now, polls show Illinois and Wisconsin leaning Democratic with seven other states too close to call.
Streb examined the Wisconsin Senate race earlier this week. The Illinois contest is between incumbent Republican Mark Kirk and Democratic U.S. Rep. Tammy Duckworth. Before we go further, Streb wants you to know the following:
"Congresswoman Duckworth was a student in my department, and received an honorary degree from NIU," he says. "I don't have a close relationship with her, but she definitely has an affiliation with the department."
When it comes to issues, it's hard to find a difference between Kirk and Duckworth. Both are harsh critics of GOP presidential nominee Donald Trump, and both demanded Senate hearings for President Obama's Supreme Court nominee, Merrick Garland.
This made Kirk stand out among Republicans. But Kirk frequently butted heads with the GOP over his pro-environment and gun-control positions.
"Kirk is definitely a moderate," Streb says. "He's socially liberal on abortion and same-sex marriage." Streb describes Kirk as the kind of Republican who can win a statewide race in Illinois. But he received an unexpected gift when running for the Senate in 2010: a Democratic opponent named Alexi Giannoulias.
"Giannoulias turned out to be not a very strong candidate," Streb says. "His family bank was taken over by federal regulators, he had all sorts of problems." The bank takeover was especially embarrassing because Giannoulias was the state's Treasurer. What's worse, he ran in a year when Republicans were energized by Tea Party conservatives. Kirk rode that wave, edging out Giannoulias and two other challengers with 48% of the vote.
Fast forward six years, and Republicans find themselves defending 24 Senate seats in a presidential election year, when Democratic voters are more likely to show up. GOP leaders looked at Kirk -- a tree hugger who got an "F" from the NRA -- and let him fend for himself while they bet on other states. That prompted Politico to declare Kirk the most endangered Republican in the country.
Kirk had lots of money, though -- $12.5 million. But according to Opensecrets.org, he spent all but $1.4 million at the end of September. Duckworth raised $14 million and still has more than $4 million on hand.
While Kirk raised his own money, Duckworth got lots of help from her party. "The Democratic Senate Campaign Committee has pumped a ton of money into this race," Streb says. "There have been a lot of advertisements. If you turn on the television, we don't see a lot from the presidential election but we've seen an awful lot from the Kirk and Duckworth campaigns, or outside groups advertising on their behalf."
Streb sees Illinois as the Democrats' Senate firewall. "If they lose Illinois, they're not going to win Pennsylvania. They're not going to win New Hampshire. They're not going to win North Carolina, Missouri or Indiana."
It's interesting to note that Kirk's opposition to Trump is a very different strategy from that of Wisconsin Sen. Ron Johnson, who's going all-in for Trump supporters. "He's tried everything to make himself look like a Democrat," Streb says of Kirk. "But if you're a Democrat in Illinois, you're going to say, 'Well, why wouldn't I just vote for a Democrat?'"
Matt Streb will join WNIJ for live Election Night updates with Jenna Dooley. Listen Nov. 8 after the polls close.