Early voting has been underway since Feb 5 the primary is Mar 7 https://www.wbez.org/government-politics/elections/2026/02/17/cook-county-board-president-democratic-primary-what-to-know-preckwinkle-reilly

Seems there's been a fair amount of coverage in Chicago media but few numbers https://www.chicagomag.com/news/whats-behind-the-cook-county-board-president-attack-ads/ cites the one poll November last year which had Preckwinkle winning by a landside, and more recent fundraising numbers (Reilly has raised a lot more).

https://en.wikipedia.org/wiki/2026_Cook_County,_Illinois,_elections#President_of_the_Cook_County_Board_of_Commissioners has the endorsements and says nobody is running in the Republican primary.

What’s Behind the Cook County Board President Attack Ads

In the battle between Preckwinkle and Reilly, a reflection of the Chicago Democratic party's two factions plays out.

https://www.chicagotribune.com/2025/10/27/cook-county-candidates-hallway-ballot/ has some info on fundraising. No indication of other major candidates.

Cook County candidates pack the hallway to get on the ballot

Dragging petition papers to get on the March ballot, candidates for Cook County offices filled the hallway beneath the county’s administrative building early Monday.