{
  "schemaVersion": "vultax-public-research-data-v1",
  "articleUrl": "https://vultax.com/research/polymarket-vs-kalshi-midterms-odds-2026",
  "datasets": [
    {
      "title": "The most-traded midterm contracts, side by side",
      "columns": [
        {
          "name": "Contract",
          "unit": null
        },
        {
          "name": "Kalshi bid / ask",
          "unit": null
        },
        {
          "name": "Polymarket bid / ask",
          "unit": null
        },
        {
          "name": "Median gap, 30 days",
          "unit": null
        },
        {
          "name": "Kalshi open interest",
          "unit": null
        },
        {
          "name": "Polymarket volume",
          "unit": null
        }
      ],
      "rows": [
        [
          "House control — Democrats",
          "85 / 86¢",
          "86 / 87¢",
          "+2.5 pts",
          "7.26M contracts",
          "$6.29M"
        ],
        [
          "House control — Republicans",
          "14 / 15¢",
          "13 / 14¢",
          "−2.5 pts",
          "12.38M contracts",
          "$4.67M"
        ],
        [
          "Senate control — Democrats",
          "48 / 49¢",
          "52 / 53¢",
          "+3.5 pts",
          "1.75M contracts",
          "$2.54M"
        ],
        [
          "Senate control — Republicans",
          "51 / 52¢",
          "48 / 49¢",
          "−3.5 pts",
          "2.65M contracts",
          "$1.76M"
        ],
        [
          "Texas — Talarico (D)",
          "50 / 51¢",
          "51 / 52¢",
          "+1.5 pts",
          "3.14M contracts",
          "$489K"
        ],
        [
          "Texas — Paxton (R)",
          "49 / 51¢",
          "48 / 49¢",
          "−2.5 pts",
          "4.48M contracts",
          "$580K"
        ],
        [
          "Maine — Jackson (D)",
          "66 / 67¢",
          "70 / 71¢",
          "+2.5 pts",
          "1.10M contracts",
          "$803K"
        ],
        [
          "Maine — Collins (R)",
          "33 / 34¢",
          "29 / 30¢",
          "−2.5 pts",
          "1.67M contracts",
          "$537K"
        ],
        [
          "Michigan — El-Sayed (D)",
          "62 / 63¢",
          "62 / 63¢",
          "+1.5 pts",
          "0.94M contracts",
          "$391K"
        ],
        [
          "Michigan — Rogers (R)",
          "38 / 39¢",
          "36 / 38¢",
          "−1.5 pts",
          "1.51M contracts",
          "$318K"
        ],
        [
          "Ohio — Brown (D)",
          "52 / 53¢",
          "51 / 52¢",
          "+1.5 pts",
          "1.09M contracts",
          "$172K"
        ],
        [
          "Ohio — Husted (R)",
          "47 / 48¢",
          "48 / 49¢",
          "−2.0 pts",
          "1.08M contracts",
          "$138K"
        ],
        [
          "North Carolina — Cooper (D)",
          "91.1 / 91.9¢",
          "92 / 93¢",
          "+0.2 pts",
          "1.18M contracts",
          "$45K"
        ],
        [
          "Iowa — Turek (D)",
          "38 / 40¢",
          "38 / 40¢",
          "−1.5 pts",
          "0.89M contracts",
          "$134K"
        ],
        [
          "Iowa — Hinson (R)",
          "61 / 62¢",
          "60 / 62¢",
          "+1.0 pts",
          "0.68M contracts",
          "$138K"
        ]
      ],
      "measurementContext": "Quotes read 18:52 UTC (Kalshi) and 18:53 UTC (Polymarket), 12 September 2026. Kalshi open interest counts contracts that pay $1; Polymarket volume is lifetime dollars traded. Polymarket prices are from its international exchange, which U.S. traders cannot use.",
      "sources": [
        {
          "label": "Kalshi public trade API — events and markets",
          "url": "https://api.elections.kalshi.com/trade-api/v2/events?series_ticker=CONTROLS&with_nested_markets=true",
          "note": "Quotes, open interest and rule text read 18:52 UTC, 12 September 2026; order books every 30 minutes, 12–13 September 2026"
        },
        {
          "label": "Kalshi public trade API — daily candlesticks",
          "url": "https://api.elections.kalshi.com/trade-api/v2/series/CONTROLS/markets/CONTROLS-2026-D/candlesticks?period_interval=1440",
          "note": "Daily closes used for the gap history"
        },
        {
          "label": "Polymarket — Which party will win the Senate in 2026?",
          "url": "https://gamma-api.polymarket.com/events/slug/which-party-will-win-the-senate-in-2026",
          "note": "Quotes and full resolution rules, read 18:53 UTC, 12 September 2026"
        },
        {
          "label": "Polymarket CLOB — price history",
          "url": "https://clob.polymarket.com/prices-history?market=113287701564209339913693347405685749986285999146352375265161592243948562084773&interval=max&fidelity=1440",
          "note": "Daily price points used for the gap history"
        },
        {
          "label": "Polymarket CLOB — order books",
          "url": "https://clob.polymarket.com/books",
          "note": "Midterm contract books, 12–13 September 2026"
        },
        {
          "label": "Polymarket — trading fees",
          "url": "https://docs.polymarket.com/trading/fees",
          "note": "Politics rate 0.04; makers pay nothing; read 2026-09-12"
        },
        {
          "label": "Kalshi — fee rounding",
          "url": "https://docs.kalshi.com/getting_started/fee_rounding",
          "note": "Trade fees are rounded up per fill"
        },
        {
          "label": "OddsShopper — Kalshi fees",
          "url": "https://www.oddsshopper.com/articles/prediction-markets/kalshi-fees",
          "note": "Third-party statement of the 0.07 taker coefficient; Kalshi's own schedule could not be retrieved"
        },
        {
          "label": "Vultax: Kalshi vs Polymarket",
          "url": "https://vultax.com/research/kalshi-polymarket-open-interest-by-category",
          "note": "The venues compared: structure, fees and open interest"
        },
        {
          "label": "Vultax: Kalshi–Polymarket arbitrage after fees",
          "url": "https://vultax.com/research/kalshi-polymarket-arbitrage-gaps-after-fees",
          "note": "How often a midterm gap cleared both venues' fees, and what it returned"
        },
        {
          "label": "Vultax: Is Polymarket legal in the US?",
          "url": "https://vultax.com/research/is-polymarket-legal-in-the-us-2026",
          "note": "Which Polymarket U.S. residents can use"
        }
      ],
      "measurement": {
        "kind": "observed",
        "temporalCoverage": "2026-08-13/2026-09-12",
        "population": "15 of 65 matched midterm contract pairs",
        "methodology": "Best bid and ask from each venue's public API; gap is the median over 30 days of Polymarket's daily price minus Kalshi's daily closing price."
      },
      "id": "side-by-side-table",
      "kind": "published-table",
      "description": "Vultax matched 65 midterm contracts across Polymarket and Kalshi. The venues usually sit 1 to 3 points apart, Polymarket prices Democrats higher in 15 of the 18 most-traded outcomes, and $50,000 on a Democratic Senate cost 1.0¢ above the price on Kalshi and 2.4¢ on Polymarket.",
      "articleUrl": "https://vultax.com/research/polymarket-vs-kalshi-midterms-odds-2026",
      "sectionUrl": "https://vultax.com/research/polymarket-vs-kalshi-midterms-odds-2026#side-by-side",
      "publishedAt": "2026-09-16",
      "modifiedAt": "2026-09-16",
      "version": "5e3685ed73b83e6503a62952850b276ea3a99834604991dc20a95cf9123da270",
      "isLive": false,
      "limitations": "Published research snapshots and calculations; these exports do not contain current quotes or live account data. Observation windows and populations are stated in the source captions and methodology. Missing metadata is unknown, not zero. Prices and order books were read from public venue data on 12 and 13 September 2026 and change continuously. Gap histories compare daily prices taken at different times of day. Price-impact figures exclude fees and assume no reaction from other traders. Not a forecast, a recommendation to trade, or financial advice.",
      "downloads": {
        "json": "https://vultax.com/research/polymarket-vs-kalshi-midterms-odds-2026/data.json?dataset=side-by-side-table",
        "csv": "https://vultax.com/research/polymarket-vs-kalshi-midterms-odds-2026/data.csv?dataset=side-by-side-table"
      }
    }
  ]
}