Polymarket
KalshiIllinois's 12th District: Bost's Real Test Was the 2024 Primary, and November Prices as a Formality
The Illinois 12th District race in 2026 is Republican Rep. Mike Bost against Democrat Julie Fortier, and the market rests on where the seat's only close contest of the last four years took place: not in a general election, which Bost won by 145,588 votes in 2022 and by 177,879 in 2024, but in the March 2024 Republican primary, which he survived by 2,735 votes. Illinois voters in the 12th decide the general election on November 3, 2026, and the live board above carries Kalshi's and Polymarket's current prices for each party.
Who Is Running in the Illinois 12th District
The State Board of Elections' website candidate list for the general election, read on October 29, 2026, has two active candidates for the 12th Congressional District: Julie Fortier for the Democrats and Mike Bost for the Republicans, both of whom filed on October 27, 2025. There is no independent or third-party name, and Polymarket's market lists Bost and Fortier beside their parties.
Bost is the incumbent. The Clerk of the House member list published October 1, 2026 records him as the Republican member for the 12th, with Murphysboro as his home town, re-elected on November 5, 2024. The same list shows him as chair of the Committee on Veterans' Affairs, with seats on the Committee on Agriculture and the Committee on Transportation and Infrastructure.
Both nominees were unopposed on March 17, 2026. Bost took 81,400 Republican primary votes and Fortier 26,791 Democratic ones, according to the Board's precinct returns, which come from election authorities in 34 counties, among them St. Clair, Monroe, Jackson, Williamson, Marion, Effingham, Coles, Alexander and Pulaski.
What the Illinois 12th District Price Rests On
The market treats the 12th as a Republican hold, and the general-election record is why. In 2022 Bost beat Democrat Homer "Chip" Markel 218,379 to 72,791, 75% to 25%, out of 291,171 votes. In 2024 he beat Democrat Brian Roberts 272,754 to 94,875, 74.19% to 25.81%, out of 367,629. Two elections with a quarter of a million or more voters each produced almost exactly the same three-to-one split, a midterm and a presidential year alike.
On October 29, 2026, Kalshi and Polymarket both had the Republican Party at 97%, and Polymarket had the cheapest Democratic Yes at 3%, against Kalshi's 4%. That price means the board gives the Democratic side about a one-in-thirty chance, and it puts the 12th among the most one-sided boards in the state in either party's direction.
The structure of the race explains why the price did not wobble even in a year when the seat was contested. In the 2024 Republican primary Bost took 48,770 votes, 51.44%, to 46,035 for Darren Bailey, 48.56%, a margin of 2,735 out of 94,805. Bailey is now the Republican nominee for governor on the Board's 2026 list, and Wikipedia's account of his career records that 2024 primary loss to Bost. The district's real competition came from inside the Republican Party, and once Bost cleared that test he won the general election by 48 points. In 2026 nobody filed against him in the primary at all.
Money points the same way. Through October 14, 2026, the close of books for the pre-general reports due October 22, the Federal Election Commission's totals for the 2025-2026 cycle showed Bost's committee with $1.62M in receipts and $845K in cash on hand, and Fortier's with $159K in receipts and $34K on hand. Fortier has raised far more than the Republican challenger in the 3rd District, but she entered the last three weeks with about one twenty-fifth of the incumbent's cash.
How the Illinois 12th District Is Trading
The 12th is a busier board on Kalshi than its price would suggest. Kalshi had matched about 9,600 contracts on it through October 29, 2026, about 6,500 of them on the Republican contract and 3,100 on the Democratic one, against about 1,560 on the 3rd District board. Polymarket had traded about $26.5K, $15.4K on the Republican side and $11.2K on the Democratic side. Each venue's volume is its own count, contracts on Kalshi and dollars on Polymarket, so the figures sit side by side rather than in one total. In the 24 hours before the board was read on October 29, 2026, Kalshi matched 223 contracts and Polymarket traded $382, so the board is still active five days before the vote.
Most of that money sits on the favorite. The heavier Republican side on both venues reads as traders buying a near-certain contract for the small return left on it, not as a bet on a close race.
The Case Against the Illinois 12th District Read
The strongest case against the read starts with the March primaries. In 2024, when Bost and Bailey were fighting for the Republican nomination, Republican primary voters outnumbered Democratic ones by more than five to one: 94,805 against 17,926. In 2026, with no contest on either side, the ratio fell to about three to one, 81,400 Republican votes against 26,791 Democratic. Democratic primary turnout rose by about half while Republican turnout fell. A reader who holds the counterread would say the board is pricing the 12th on the last two Novembers and ignoring a shift in who is turning out.
The counterread also points at money. Fortier had raised $159K by October 14, 2026, for a seat where the Democratic nominee has not topped 26% in either of the last two elections, and a funded challenger is the condition under which a lopsided seat starts to move.
The price has an answer to both. A three-to-one primary ratio is the same ratio Bost won by in the 2022 and 2024 general elections, so the primary that looks like a Democratic gain lands exactly on the district's existing lean. And primary turnout follows the contests at the top of each ballot, which changed between 2024 and 2026. To win, Fortier would need to roughly double the Democratic share of the last two general elections in a single cycle.
What Changes the Illinois 12th District Read
The read is that the 12th's real test was the 2024 primary and that the general election is a formality. It would be wrong if the November count breaks from the three-to-one pattern. The dated tests:
• Early voting: Early voting at permanent polling places opened October 19, 2026 and runs through November 2; a mail-ballot application sent by mail had to reach the election authority by October 29.
• Election night: Polls are open from 6 a.m. to 7 p.m. on November 3, 2026, and each of the district's county election authorities reports its own returns.
• Uncounted ballots: Each election authority must post its uncounted ballots within 48 hours of the polls closing, by November 5, 2026; a Fortier share well above the 25% to 26% of the last two Democratic nominees would be the first sign the read is wrong.
• Mail ballots and the canvass: Mail ballots postmarked by Election Day are counted through November 17, county canvasses close by November 24, and the State Board of Elections proclaims the results by December 4, 2026.
• Kalshi's settlement: Kalshi's contracts settle on the party of the member sworn in for the term beginning in 2027, with an expected settlement of January 4, 2027.
When the Illinois 12th District Market Resolves
Kalshi lists the race as its KXHOUSERACE-IL12-26 event. Its Republican contract, labeled with Bost's name, resolves Yes if the House member sworn in for IL-12 for the term beginning in 2027 is a member of the Republican Party, and its Democratic contract, labeled with Fortier's name, on the same test for the Democratic Party; both are eligible for accelerated determination after a consensus of media organizations projects the winner. Polymarket's IL-12 House Election Winner market resolves according to the party of the candidate who wins the seat in the November 3, 2026 midterm elections, as indicated by a consensus of credible reporting, and on the Federal Election Commission's official results if there is ambiguity. Because both settle on party, a Republican winner pays the Republican side however the count is called.
Related Illinois Markets
The top of the same ballot is priced on the Illinois governor odds page, where Bailey is the Republican nominee, and the Illinois Senate odds page. The other Republican-held Illinois seats with their own pages are the Illinois 15th District odds and the Illinois 16th District odds, and the Illinois 3rd District odds show the same one-sided pricing for a Democratic incumbent. The Illinois elections page lists every race on the ballot.
