Polymarket vs Kalshi Midterm Odds: 65 Matched Contracts and What a $50,000 Bet Moves

Polymarket and Kalshi mostly agree on the 2026 midterms. Across 65 contracts that exist on both venues, from control of each chamber to 31 individual Senate races, the median pair's daily prices sat 1.4 points apart over the last 30 days. On the questions that matter most on 12 September, they told the same story: Democrats around 85 to 87 cents to win the House, the Senate a coin flip, Texas and Ohio close, Maine and Michigan leaning Democratic.

Where they differ, the difference has a direction. In 15 of the 18 most-traded outcomes, Polymarket priced the Democrat higher or the Republican lower than Kalshi over the past month. A Democratic House was dearer on Polymarket on 319 of 379 days since August 2025. Vultax did not measure why; the venues have different users, fees and rules.

Money moves the headline contracts very little: in 58 readings of both venues' order books on 12 and 13 September, a $50,000 bet on a Democratic Senate cost 1.0 cent a contract above the midpoint on Kalshi and 2.4 cents on Polymarket. Races and longshots sit on thinner books, especially on Polymarket, and thin books are part of why few gaps between the venues pay, as the arbitrage study measured.

The table lists the most heavily traded contracts on Kalshi, those with open interest above 500,000 contracts; Kalshi files Ohio as a special election. Bid and ask are the best prices to sell and buy a Yes contract. The gap is Polymarket's daily price minus Kalshi's, as a median over the 30 days to 12 September; positive means Polymarket was higher.

On the chamber-control contracts the pattern is long-running. Since 9 August 2025, the first day both venues had a daily price, Polymarket priced a Democratic House higher than Kalshi on 319 of 379 days, by 1.85 points on average, and a Democratic Senate higher on 234 of 358 days, by 1.05 points. Over the last month the Senate gap widened: Kalshi's Democratic Senate price rose from 41 to 49 cents between 29 June and 12 September while Polymarket's rose from 42.5 to 52.5.

The race contracts mostly follow suit. Of the 18 outcomes with more than 500,000 contracts of Kalshi open interest, Polymarket was higher on the Democrat or lower on the Republican in 15. Iowa went the other way on both candidates, and South Carolina showed no gap. A consistent lean like this is what you would expect if the two venues draw different traders, but a price gap alone cannot say whose traders are right.

Most of the time, not far. The chart shows the average absolute difference between the venues' daily prices over the last 30 days, for the more heavily traded side of each contract. Chamber control and Maine ran about 3 points apart; North Carolina, where Roy Cooper trades above 90 cents on both, ran under 1. For comparison, Polymarket's politics fee on a trade at 50 cents is 1 cent per contract and the reported Kalshi taker fee is 1.75 cents, so gaps of 1 to 3 points are close to what it costs to act on them.

Walking both venues' order books gives the fuller answer: over 28 hours on 12 and 13 September, 10.1% of 13,440 readings of 40 matched contracts cleared both venues' fees at the size on offer, for returns under 1% on capital held until the result is called. The arbitrage study sets out which contracts and for how long.

A hedge across venues is only a hedge if both contracts pay on the same facts. They mostly will, but the rulebooks differ in ways that matter at the edges.

• The 2029 trap - Kalshi's Georgia and North Carolina pages also carry contracts for the Senate term beginning in 2029, elected in 2028. Matching those against Polymarket's 2026 races produced apparent gaps of 25 to 31 points; they were different elections, not mispricing.
• Party or person - a Kalshi contract labelled with a candidate pays on the party; Polymarket's pays on the candidate who wins. If a nominee is replaced before November, the two can split.
• Timing - Polymarket settles when three networks call a race; Kalshi's fallback is the swearing-in or the chamber's organisation in 2027, so the same position can tie up money for different lengths of time.

A price on a prediction market is only as solid as the orders behind it. Every 30 minutes from 18:51 UTC on 12 September to 23:21 UTC on 13 September 2026, 58 readings in all, Vultax filled a $1,000, $10,000 and $50,000 buy of Yes against each venue's asks for the same 65 contracts, level by level. The figure reported is the average price paid minus the quoted midpoint, in cents per contract, as a median over the readings where the order filled; fees are not included and would add to every figure.

Below the headline contracts, Kalshi's books were usually the deeper: a $10,000 buy cost less on Kalshi in 25 of the 30 contracts where both venues could fill it, and $50,000 on James Talarico in Texas cost 0.8 cents above the price on Kalshi against 6.6 cents on Polymarket, whose book could fill it at all in only 16% of readings.

Each cell shows Kalshi first and Polymarket second. Prices are medians over the 58 readings.

• Headline contracts are deep on both venues - House and Senate control absorbed $50,000 for no more than 2.6 cents on either venue.
• Races are deeper on Kalshi - the $50,000 order filled in at least 90% of readings for 18 contracts on Kalshi and 13 on Polymarket, out of 65.
• The same race can differ sharply - a $10,000 buy of Josh Turek in Iowa cost 1.4 cents on Kalshi and 10.3 cents on Polymarket.

The quote on a low-priced candidate usually sits on very little. For the seven Polymarket candidates below whose books could absorb $10,000 in at least nine readings out of ten, even $1,000 paid 1.7 to 52 cents above the midpoint, and $10,000 paid 24 to 69 cents above it. A buyer of that size would pay an average of 33 to 70 cents for candidates the screen shows at 1 to 9 cents.

Depth changes what a price move means. On House or Senate control, a jump of several points needs hundreds of thousands of dollars of orders or a real change in expectations; on a thin race or a longshot, it can be a single trader. Vultax's page for Polymarket's Senate control contract shows the same readings live: its spread, the liquidity resting in its book and its latest trades.

• Check the book behind a quote - the price a screen shows is the midpoint of the best orders, not the price of any meaningful size.
• Compare venues at your size - the cheaper venue for $1,000 is not always the cheaper venue for $50,000.
• Treat longshot spikes with care - on a book that holds a few hundred dollars near the quote, one order can move the odds by tens of points.

Kalshi contracts came from its public events and candlestick endpoints for House control, Senate control and every Senate race series, keeping only events for terms beginning in 2027. Polymarket contracts came from its public events and price-history endpoints for markets tagged midterms. Pairs were matched on chamber or state, then on party label or candidate surname, and checked by reading both rule texts; Alaska, Kentucky, Louisiana and Nebraska had no clean match. Ohio and Florida pair Kalshi's special-election series with Polymarket's race markets for the same seats.

Daily gaps compare Polymarket's daily price point with Kalshi's daily closing price on the same UTC date, which are not taken at the same second. Kalshi's fee coefficient is the rate reported by independent fee trackers, because Kalshi's fee schedule could not be retrieved from our environment; its API lists these series under its standard quadratic fee.

Order books for the price-impact figures were read from Kalshi's public trade API and Polymarket's public order-book endpoint to 20 price levels on each venue, every 30 minutes over a weekend; a buy of Yes on Kalshi fills against the Yes asks implied by its No bids. An order that could not be filled within those levels counts as not filled, although the book may hold more further out, at prices far from the quote. The cost figures exclude both venues' fees, which Kalshi rounds up, assume no reaction from other traders, and describe mid-September books that change as the campaign develops.

Polymarket prices are from its international exchange, which U.S. residents cannot use; see where U.S. law stands. For how the two venues compare in structure, fees and open interest, see Kalshi vs Polymarket. Nothing here is a forecast or a recommendation to trade. Challenge a figure at [email protected]; material changes are logged in the changelog.