{
  "schemaVersion": "vultax-public-research-data-v1",
  "articleUrl": "https://vultax.com/research/polymarket-vs-kalshi-midterms-odds-2026",
  "datasets": [
    {
      "title": "The contracts are not written the same way",
      "columns": [
        {
          "name": "",
          "unit": null
        },
        {
          "name": "Kalshi",
          "unit": null
        },
        {
          "name": "Polymarket",
          "unit": null
        }
      ],
      "rows": [
        [
          "Chamber control",
          "Pays if the party \"has won control\"; may be settled early on a consensus of media calls, otherwise by the party of the Speaker (House) or President pro tempore (Senate) on 1 February 2027",
          "Counts seats: a majority of voting seats, or half plus the Vice Presidency; independents count with the party they caucus with or announce they will; may stay open until 4 January if caucus intentions are unclear"
        ],
        [
          "Senate races",
          "Markets carry a candidate's name but pay if \"a representative of the Republican party\" (or Democratic) \"is sworn in\" for the term beginning in 2027; early settlement on media consensus is allowed",
          "Pays the winner of the 2026 election including any runoff, once the Associated Press, Fox News and NBC all call it; independents do not count as a party's candidate"
        ],
        [
          "Which election",
          "Also lists contracts for the Senate term beginning in 2029 in the same state series",
          "Lists 2026 races under \"Senate Election Winner\""
        ],
        [
          "Fee to take liquidity",
          "0.07 × p × (1 − p) per contract as reported by fee trackers, rounded up",
          "0.04 × p × (1 − p) per share on politics markets; makers pay nothing"
        ]
      ],
      "measurementContext": "Rule text read from each venue's public market data on 12 September 2026. Kalshi's full rules and fee schedule are longer than the summaries its API returns.",
      "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"
        },
        {
          "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 — 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, the odds gap measured",
          "url": "https://vultax.com/research/kalshi-vs-polymarket-odds-gap-measured",
          "note": "Matched Fed contracts"
        },
        {
          "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": "reference",
        "temporalCoverage": "2026-09-12",
        "population": null,
        "methodology": "Quoted from Kalshi rules_primary and rules_secondary fields and Polymarket market descriptions."
      },
      "id": "rules-table",
      "kind": "published-table",
      "description": "Vultax matched 65 midterm contracts across Polymarket and Kalshi, from House and Senate control to 31 Senate races. The venues usually sit 1 to 3 points apart, Polymarket prices Democrats higher in 15 of the 18 most-traded outcomes, and 4 of 40 pairs cleared fees at depth.",
      "articleUrl": "https://vultax.com/research/polymarket-vs-kalshi-midterms-odds-2026",
      "sectionUrl": "https://vultax.com/research/polymarket-vs-kalshi-midterms-odds-2026#rules",
      "publishedAt": "2026-09-16",
      "modifiedAt": "2026-09-16",
      "version": "f175155a61264475c63ca527b6a1e60fdb5d8346566cb96543e4912eac070fd4",
      "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 September 2026 and change continuously. Gap histories compare daily prices taken at different times of day. Fee estimates use published or reported rates and exclude the cost of capital and settlement risk. 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=rules-table",
        "csv": "https://vultax.com/research/polymarket-vs-kalshi-midterms-odds-2026/data.csv?dataset=rules-table"
      }
    }
  ]
}