U.S. Senate control

Both contracts pay on which party controls the Senate after the November 3 election, and the two settlement rules give the same answer in every seat outcome except a chamber organized by an independent's vote, so a lasting gap is either a difference in what the contracts pay on or a cost of holding both. After the October 11 entry closed, the gap ran from 0 to 2 points on every board from October 12 to 24, with Kalshi lower on all but October 23, when both were at 67.5%. On October 25 Kalshi fell from 65.5% to 62.5% and Polymarket from 67.5% to 66.5%, a 4-point gap. Today Kalshi rose to 63.5% and Polymarket fell to 64.5%. Kalshi's seat-count market, summed at 51 seats and above, went from 66.4% to 64.8% over the same day; on both days it ended within half a point of Polymarket's price.

• Settlement wording: Kalshi settles on the party of the President pro tempore on February 1, 2027; Polymarket on a party holding more than half the voting seats, or half plus the Vice Presidency, falling back to the Majority Leader's party if that is ambiguous and to "Other" if the leader caucuses with neither party. The rules part only in a chamber organized by an independent's vote.
• Quoting: the board takes Kalshi's figure as the midpoint of a 63-64 quote and Polymarket's as the outcome price its public API publishes, with no bid or ask recorded, so part of the gap may sit inside Polymarket's quote.
• Flow on one contract: the October 25 widening came from Kalshi's control contract, which fell 3 points while Kalshi's seat-count market rose and the probability its thirteen seat contracts imply fell under a point; today's narrowing came mostly from Polymarket's 2-point fall. The move may sit with each contract's traders rather than with news.

Both contracts pay on which party controls the Senate after the November 3 election, and in almost every seat outcome the two settlement rules give the same answer, so a lasting gap is either a difference in what the contracts pay on or a cost of holding both. The gap ran from 0 to 2 points on the boards from October 2 to 10, with Kalshi lower on seven and level on two, and reached 3 today as Polymarket rose from 62.5% on October 9 to 63.5% while Kalshi held at 60.5% for a third board.

• Settlement wording: Kalshi settles on the party of the President pro tempore on February 1, 2027; Polymarket on a party holding more than half the voting seats, or half plus the Vice Presidency, falling back to the Majority Leader’s party if that is ambiguous and to “Other” if the leader caucuses with neither party. The rules part only in a chamber organized by an independent’s vote.
• Quoting: the board takes Kalshi’s figure as the midpoint of a 60-61 quote and Polymarket’s as the outcome price its public API publishes, with no bid or ask recorded, so part of the gap may sit inside Polymarket’s quote.
• A standing offset: Kalshi has been at or below Polymarket on all ten boards, through a 4-point fall in Kalshi’s price since October 5, so the difference may lie in who trades each venue rather than in news; today’s widening came with no move on Kalshi.

Both models take the same public polling and the same seat map, and each one's control number sits inside the range its own seat probabilities imply. So the gap is not about how seats combine; it is the sum of seat-level differences. Kansas is widest: FiftyPlusOne's number fell from 51.2% to 48.7% in its October 26 reading, against DDHQ's 23% and Kalshi's 39.5%, both unchanged since October 24. Against the five-aggregator poll average of Hamilton +0.8, FiftyPlusOne takes 1.0 point off his lead, Kalshi 2.5 and DDHQ 5.6. In Ohio the poll average is Brown +3.8; DDHQ's model takes 3.6 points off his lead, FiftyPlusOne, down from 64.5% to 62.7%, takes off 1.7, and Kalshi, unchanged at 67.5%, takes off 0.9. In Maine, where the poll average is Jackson +2.0, DDHQ takes 1.5 points off his lead and FiftyPlusOne adds 1.2; Kalshi fell from 60.5% to 59.5% and takes 0.4 off. Expected Democratic seats are 50.5 for DDHQ and 51.5 for FiftyPlusOne, either side of the majority line. Kalshi (63.5%, up 1 point since October 25) and Polymarket (64.5%, down 2) are still between the two models. FiftyPlusOne's control number fell from 73.5% to 72.4% in the same reading, which narrowed the spread from 19.5 points to 18.4.

• Maine is where structure and polling point opposite ways: incumbents in Collins's position have lost 8 of 9 midterm races since 2006, but her 2020 result beat the final polling average by 12 points.
• Ohio and Kansas turn on how far to mark down a Democratic poll lead in a state Trump carried. The record runs from 2 to 3 points in the contested midterm races of 2018 and 2022 to 5.3 across all Republican-state midterm races since 2014. Brown adds a wrinkle: in 2024 he ran 7.6 points ahead of the top of the ticket and still lost by 3.6.
• Timing: DDHQ's reading is dated October 2. Its page showed no newer one when read on October 3 and 4, and the daily reader has recorded no newer reading on any day from October 5 to 26, so a newer DDHQ number, if one exists, is not on the board. FiftyPlusOne's is dated October 26. Every poll published in between, including New York Times/Siena surveys of Alaska and Ohio fielded through October 1 and YouGov's Kansas survey fielded through October 4, may be in one and not the other.
• A third model sits with FiftyPlusOne. Silver Bulletin's Deluxe forecast, read once on October 4 from its subscriber page and not a board source, has control at 72.8% and publishes projected margins, so its markdown can be read directly: it adds 0.8 points to its own poll average in Maine and takes 2.1 off in Kansas and 0.4 off in Ohio, nearer FiftyPlusOne's inferred markdowns than DDHQ's in all three. Its Classic version, without expert ratings, has control at 70.0% and differs by 0.2 to 0.7 points in those seats, so expert ratings, which FiftyPlusOne also uses, are worth 2.8 points of control inside Silver Bulletin's model, against a gap of 18. Agreement between two models identifies DDHQ as the one that differs and leaves open which is closer to the result.

All three numbers price the same event, a Democratic majority. The seat contracts are combined at the correlation Kalshi's own four-seat joint contract implies, 0.84, from 0.88; that contract was 56.5% on October 25 and 53.5% today. Across 0.69 to 0.95 the seats give 60.5% to 61.7%, with 51.1 expected Democratic seats. Since October 25 control rose from 62.5% to 63.5% while the seats fell slightly: Michigan 3 points to 65.5%, Georgia 1 to 94.65% and Maine 1 to 59.5%, against South Carolina up 4 to 24% and Florida and Nebraska up 2 each to 12.5% and 26.5%. The seat-count market went from 66.4% to 64.8%, so the two Kalshi markets that price the chamber directly moved toward each other after moving apart on October 25. It was 2 to 4.8 points above control from October 4 to 11, 2.1 on October 17, 2.4 and 2.2 on October 22 and 23, 0.4 on October 24 and 3.8 on October 25. The entry closes when both legs are under 2 points on two consecutive boards.

• Seats outside the count: independents in Montana and Idaho, and Osborn in Nebraska at 26.5%. Any of them caucusing with Democrats adds a seat the thirteen contracts do not carry.
• Settlement rule: the control contract pays on the party of the President pro tempore, so a 50-49-1 Senate organized with Osborn's vote would resolve for Democrats without a 51st seat.
• Quoting and flow: the seat-count figure is summed from the midpoints of several bucket contracts, normalized to sum to 1, so wide quotes in the buckets can move it by points with no trade; the control contract is the headline market and may move on attention alone.