We certainly used to be circa the late 1980s, when Chicagoland made up a smaller percentage of the state's population, Downstate Illinois was a bit more thriving and the state was more in line with the nation politically (we voted for Bush in 1988 by just a couple points less than the nation). More specifically, my hometown of Peoria was long described as a perfect little microcosm WITHIN such a representative state! While the phrase "Will it play in Peoria?" did originally refer to our musical/play scene being one of the best springboards to Broadway, it indeed came to refer to Peoria being a great sample size of the American market place for products.
From the Pew Religious Landscape Survey:
National
Evangelical 25%
Mainline Protestant 15%
Catholic 21%
Non-Christian 6%
None 23%
...
Illinois
Evangelical 20%
Mainline 16%
Catholic 28%
Non-Christian 6%
None 23%
It is worth noting that Pew categorizes the Mainline churches as the Seven Sisters plus other theologically moderate-to-liberal churches (e.g., the Reformed Church in America) that are part of ecumenical organizations, and then they just lump all conservative denominations into their "Evangelical" category. This is problematic, as groups like the Lutheran Church-Missouri Synod are not "Evangelical" in any way, shape or form ... they're just conservative. They practice infant baptism, have formal liturgy, place a high emphasis on the sacraments and church tradition, etc. They are in no way "Born Again." Thus, many denominations researchers (such as Joshua from the Ready to Harvest YouTube channel) prefer an additional category of "Confessional Protestant," which describes the more conservative denominations from the more historic/classical Protestant traditions.
If you moved those groups out of the "Evangelical" category and left that category to refer more to what we think of as "Evangelical" like Baptists, Pentecostals, Non-Denominational, etc., Illinois' numbers would look like this:
16% MAINLINE PROTESTANT
3% United Methodist Church
2% Evangelical Lutheran Church in America
2% American Baptist Churches USA
1% Presbyterian Church (USA)
1% Episcopal Church
1% United Church of Christ
6% Other
15% EVANGELICAL PROTESTANT
2% Southern Baptist Convention
2% Independent Baptists
2% Christian Churches and Churches of Christ
1% Assemblies of God (Pentecostal)
5% Non-Denominational
3% Other
5% CONFESSIONAL PROTESTANT
3% Lutheran Church-Missouri Synod
1% Presbyterian Church in America and other Presbyterian denominations
1% Anglican, Methodist, Congregationalist, etc. confessional denominations
TL;DR
If you are speaking of what everyone would think of as "Mainline" and "Evangelical" denominations, there are more Mainline Protestants in Illinois. The problem is lumping groups that are not Evangelical in with the Evangelical category for Pew.
I think you're misunderstanding the definition of "Evangelical". Certainly, many Evangelical churches are of the "low-church" variety, like Baptists, that believe in credobaptism and have a more symbolic view of sacraments. But, that's not what defines Evangelical. "Evangelical" means believing in the need to evangelize (due to believing that all non-Christians go to Hell). Conservative denominations like the PCA, LCMS, ACNA, and Methodist offshoots believe that. That's why they're classified as "Evangelical".
