Midterms eve: Kalshi and Polymarket line up on Senate control as Kalshi's balance-of-power notional rises

US midterm polling day is tomorrow, Tuesday 3 Nov. On the eve of the vote, Kalshi and Polymarket are quoting the Senate control contract at the same price.

Snapshot time. All figures are from each venue’s public API at 06:34 GMT (06:34 London time) on Monday 2 Nov 2026. Prices move continuously, so later quotes will differ. Changes are against The Midpoint’s own checks of the same endpoints at 06:23 GMT on Friday 30 Oct, so they cover about three days. Polymarket figures are from its international platform’s Gamma API.

Definitions (our calculations).

• Prices (¢): US cents per contract (Kalshi) or share (Polymarket) that pays $1 if the outcome occurs and nothing otherwise. A 72.5¢ quote means a price of $0.725. The figures shown are mid quotes, calculated by The Midpoint as (best bid + best ask) / 2. They are market quotes, not probabilities or forecasts.
• Notional: lifetime traded volume counted at $1 face per contract or share (Kalshi volume_fp; Polymarket Gamma volume, which on our check of its public trade records tracks shares traded). It is not the cash paid. A contract bought at 6¢ adds $1 of notional but costs 6¢.
• Single-cent contracts: contracts with a mid quote below 10¢ at the snapshot.

Senate: the venues line up

Contract	Kalshi mid	Change (pts)	Polymarket mid	Change (pts)
Senate: Democratic	72.5¢	+5.0	72.5¢	+4.0
Senate: Republican	27.5¢	−5.0	28.5¢	−3.0

On 30 Oct, Kalshi quoted the Democratic Senate contract at 67.5¢ and Polymarket at 68.5¢. Both now show a 72¢ bid and a 73¢ ask, so the one-point gap between the venues has closed on the Democratic leg. On the Republican leg Polymarket’s mid is one point higher than Kalshi’s (28¢ bid, 29¢ ask against 27¢ and 28¢), so Polymarket’s two Senate mids add up to 101¢.

House: a gap of under a point

Contract	Kalshi mid	Change (pts)	Polymarket mid	Change (pts)
House: Democratic	93.65¢	+1.7	94.5¢	+1.0
House: Republican	6.45¢	−1.7	5.5¢	−1.5

The House contracts show the only cross-venue gap of note in the single-chamber markets: 0.85 points on the Democratic leg and 0.95 points on the Republican leg. Kalshi’s API lists a 0.1¢ tick below 10¢ and above 90¢ (1¢ in between), and its House book was 93.6¢ bid and 93.7¢ ask. Polymarket’s House book was 94¢ bid and 95¢ ask.

Balance of power

Outcome	Kalshi mid	Change (pts)	Polymarket mid	Change (pts)
Democratic House, Democratic Senate	70.5¢	+4.0	71.5¢	+2.0
Democratic House, Republican Senate	24.5¢	−2.0	23.5¢	−1.0
Republican House, Republican Senate	6.4¢	−1.55	5.5¢	−1.0
Republican House, Democratic Senate	0.45¢	−0.3	0.65¢	−0.05

Polymarket’s balance-of-power event also lists an “Other” outcome, quoted at 0.1¢ ask with no bid. Its $6.15m of notional is included in the Polymarket event total below.

Where the notional sits

Lifetime notional ($1 face, not cash) at 06:34 GMT on 2 Nov, with the change since 06:23 GMT on 30 Oct:

Event	Kalshi	Change	Polymarket	Change
House control	$95.54m	+$13.52m	$37.43m	+$6.23m
Senate control	$26.96m	+$4.64m	$15.78m	+$3.93m
Balance of power	$93.89m	+$26.14m	$51.77m	+$6.75m
Total	$216.39m	+$44.30m	$104.99m	+$16.92m

Kalshi’s three control events carry about twice Polymarket’s notional. Balance of power had Kalshi’s largest increase since 30 Oct, and every one of its four outcomes added at least $3.2m.

On Kalshi, most of the House and balance-of-power notional sits in single-cent contracts (mid below 10¢) that pay out only if Republicans win the House: the House Republican contract (6.45¢), “Republican House, Republican Senate” (6.4¢) and “Republican House, Democratic Senate” (0.45¢). Together they hold $136.81m, or 72% of Kalshi’s $189.43m of House and balance-of-power notional (63% of its $216.39m across all three events). By event:

• House: the Republican contract has $68.90m of lifetime notional against $26.64m for the Democratic one. It accounted for $11.35m of the House event’s $13.52m increase since 30 Oct. Its open interest is 52.84m contracts, against 18.19m for the Democratic contract.
• Balance of power: “Republican House, Republican Senate” ($34.91m) and “Republican House, Democratic Senate” ($33.00m) together hold $67.91m, or 72% of the event. “Democratic House, Democratic Senate” holds $15.03m.
• Senate: the Republican contract holds $16.22m against $10.74m for the Democratic one. At 27.5¢ it is not a single-cent contract.

Notional counts each contract at $1 face whatever its price, so for single-cent contracts it overstates the cash behind them. A contract trading at 6¢ adds about 16 times more notional per dollar spent than one trading at 94¢. These figures say more about the number of contracts traded than about the money behind each outcome.

In the 24 hours to the check, Kalshi’s API reported $5.21m of House notional (of which $4.34m in the Republican contract), $1.90m in the Senate and $8.90m in balance of power. Polymarket’s Gamma API reported $2.39m, $2.20m and $2.51m for the same three events.

Both venues showed one-cent spreads on the Senate contracts and on the “Democratic House, Democratic Senate” balance-of-power contract. Displayed size differed on each side of the book. On Kalshi’s balance-of-power “Democratic House, Republican Senate” contract there were offers for about 1.57m contracts at 25¢, against bids for about 30,000 contracts at 24¢.

Not the same contract

The quotes line up, but the contracts settle differently.

• Kalshi: the Senate and House contracts “may be determined early based on a consensus of media calls projecting control”. Otherwise they are settled on the party of the President pro tempore of the Senate, or of the Speaker of the House, on 1 Feb 2027. The balance-of-power contracts are combinations: each pays out only if both its House and its Senate components occur, and resolves No if either component resolves No. Each component follows the same control rules, including early determination. All three events have a scheduled close of 1 Feb 2027.
• Polymarket: the contracts resolve on which party wins a majority of the chamber’s voting seats. For the Senate, if no party has a majority, control goes to the party that wins half the voting seats and holds the vice presidency. If the outcome is still ambiguous, the fallback is the party of the first President pro tempore (Senate) or the first elected Speaker (House) of the new Congress. Resolution sources are official state and congressional information and “a consensus of credible reporting”. The API lists an end date of 04:59 GMT on 4 Nov, but the rules allow a market to remain open until as late as 4 Jan 2027 if the caucus intentions of independents cannot be determined.

Why it matters

The two venues quote the headline Senate contract at the same price with one-cent spreads, the day before polling. The public APIs do not show who is trading or whether the same participants are active on both venues. Kalshi’s operator, KalshiEX LLC, is a CFTC-designated contract market (the CFTC granted the designation on 4 Nov 2020). The Polymarket quotes here come from its international platform.

The notional figures need care. Kalshi’s larger totals are concentrated in single-cent contracts, where $1-face notional overstates the cash committed. The different settlement rules matter most in a close or disputed result. Kalshi can settle early on a consensus of media calls or wait until 1 Feb 2027, while Polymarket’s rules depend on seat counts and, for the Senate, the vice presidency.

What we are watching

• Quotes and cross-venue gaps as results are reported on 3 and 4 Nov.
• Whether Kalshi settles early on media calls and when Polymarket resolves.
• Notional and open interest once control of each chamber is determined.

Sources and check times

All market data checked by The Midpoint on 2 Nov 2026 at 06:34 GMT (London time). Baseline figures are from our checks of the same endpoints at 06:23 GMT on 30 Oct 2026. Mid quotes, changes, notional totals and percentage shares are The Midpoint’s calculations from these responses.

• Kalshi trade API, event CONTROLS-2026 (Senate control): https://api.elections.kalshi.com/trade-api/v2/events/CONTROLS-2026?with_nested_markets=true

• Kalshi trade API, event CONTROLH-2026 (House control): https://api.elections.kalshi.com/trade-api/v2/events/CONTROLH-2026?with_nested_markets=true

• Kalshi trade API, event KXBALANCEPOWERCOMBO-27FEB (balance of power): https://api.elections.kalshi.com/trade-api/v2/events/KXBALANCEPOWERCOMBO-27FEB?with_nested_markets=true

• Polymarket Gamma API, “Which party will win the Senate in 2026?”: https://gamma-api.polymarket.com/events?slug=which-party-will-win-the-senate-in-2026

• Polymarket Gamma API, “Which party will win the House in 2026?”: https://gamma-api.polymarket.com/events?slug=which-party-will-win-the-house-in-2026

• Polymarket Gamma API, “Balance of Power: 2026 Midterms”: https://gamma-api.polymarket.com/events?slug=balance-of-power-2026-midterms

• CFTC Press Release 8302-20, “CFTC Designates KalshiEX LLC as a Contract Market”, 4 Nov 2020: https://www.cftc.gov/PressRoom/PressReleases/8302-20

• Federal Election Commission, election date (Tuesday 3 Nov 2026): https://www.fec.gov/introduction-campaign-finance/election-results-and-voting-information/

Contract rules are quoted from the rules_primary and rules_secondary fields (Kalshi) and the market description field (Polymarket) in the same API responses.