The dividing lines between the Chicago area and the rest of Illinois can be stark. In 2024, Kamala Harris carried Cook County and most of its surrounding suburbs, while Donald J. Trump prevailed in large swaths of the rest of the state, which has far lower population density.
When it comes to Democratic primaries, these political fissures play out somewhat differently. There are some counties to keep an eye on as results begin to be announced on Tuesday night in the race to replace Senator Richard J. Durbin, who is retiring.
Cook County
Chicago and its immediate suburbs
Cook is the state’s most populous county and is home to Chicago. It plays an outsize role in all statewide elections in Illinois and will be especially important on Tuesday night because of its large Democratic voter base. Historically, about 60 percent of the Democratic primary vote has come from Cook, with about 40 percent coming from Chicago alone.
The diversity of the neighborhoods in Cook County, particularly Chicago, means that many of the demographic divides in the race will be evident here. Majority Black areas could vote differently from majority-white or majority-Hispanic ones, for example. Unfortunately, it may be hard to understand those immediately because the level of detail that will be available on Tuesday night is limited. Chicago is expected to begin reporting citywide results live shortly after polls close at 8 p.m. Eastern, but it does not expect to have more detailed breakdowns until well after midnight. Suburban Cook County — the parts of Cook outside of Chicago — is expected to report live results for the entire area on Tuesday night, but further breakdowns will not be available for days.
Still, there are crucial data points to watch. For Lt. Gov. Juliana Stratton, the path to victory runs through Chicago, and she is favored to come out ahead there. But a key question is the extent to which she and Representative Robin Kelly, both Black women, will split the vote in the majority Black neighborhoods in the city’s South and West sides.
Ms. Stratton was born and raised in Chicago and currently lives there, and Ms. Kelly’s House district contains parts of its South Side.
A divided Black electorate would benefit Representative Raja Krishnamoorthi, who most polls show is in a close race with Ms. Stratton. Mr. Krishnamoorthi is expected to do better overall in the Cook suburbs. His House district partially encompasses the predominantly white areas to the northwest (he lives in Schaumburg). Ms. Kelly’s district includes the more diverse areas in the southern part of the county.
Collar Counties
DuPage, Kane, Lake, McHenry, Will
About two in 10 of the votes cast in recent Democratic primaries have come from the five so-called collar counties that surround Cook.
Mr. Krishnamoorthi is expected to lead in this area. His district encompasses parts of DuPage and Kane counties, and the demographics of the collars — predominately white, wealthy and suburban — are broadly seen as favoring him. Just how well Mr. Krishnamoorthi performs in the collar counties will be evidence of his broader strength in suburban areas.
St. Louis Suburbs
St. Clair, Madison
Mr. Durbin, whose impending retirement created the open seat, lives in Springfield but is a native of East St. Louis in St. Clair County, which is one of the 96 “downstate” counties outside Cook and the collars. Last year, upon announcing his decision not to seek re-election, Mr. Durbin encouraged his successors to “discover downstate.”
The Metro East region — which includes St. Clair, Madison and Monroe counties and is named for its proximity to St. Louis, Mo. — is one of the more populous downstate areas, and all three candidates have campaigned there.
St. Clair and Madison could be particularly interesting, as they will most likely start posting votes at the precinct level once polls close. Voting patterns in the majority Black cities of East St. Louis and Venice could offer early indications of how Black voters are leaning.
Notable Counties With Universities
Champaign, McLean, DeKalb, Peoria
Ms. Stratton attended the University of Illinois at Urbana-Champaign, in Champaign County. Bernie Sanders won Champaign in the 2020 Democratic presidential primary, when every other county in the state went for Joe Biden.
A similar pattern played out in the 2018 Democratic primary for governor, which was the last time Illinois had competitive statewide Democratic primaries. That year, Daniel Biss, then a progressive state senator, won Champaign and McLean County, which is home to Illinois State University, while JB Pritzker, the billionaire heir and businessman who is now the governor of Illinois, won virtually everywhere else. That could be good for Ms. Stratton, who is seen as more progressive than Mr. Krishnamoorthi.
DeKalb County is home to Northern Illinois University, where Ms. Kelly earned a Ph.D. in political science. Along with McLean, DeKalb is expected to report precinct-level results tonight. Though these areas are not expected to make up a large share of the electorate, the results in university towns could provide insight into young voters and their preferences among Democrats.
Additionally, Mr. Krishnamoorthi and Ms. Kelly both have ties to the city of Peoria, in the county of the same name. Mr. Krishnamoorthi, who immigrated to the United States from India with his family as an infant, grew up there, while Ms. Kelly went to Bradley University.
Alex Lemonides and Caroline Soler contributed reporting.
Christine Zhang is a Times reporter specializing in graphics and data journalism.
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