One week out, prediction markets see Democrats as favored to sweep Congress

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The 2026 U.S. midterm election will be held a week from Tuesday, with the Republican Party looking to defend majorities in both the House and Senate, though Democrats are seen as two-to-one favorites to flip both chambers blue on prediction markets.

Speculators on platform Kalshi place a 63% chance that Democrats win control of both the House and Senate. Those odds have stayed relatively stable since the beginning of October. Meanwhile, there's just under a 10% chance that Republicans sweep both chambers, and a 29% chance Democrats win the House while the GOP maintains its majority in the Senate.

Contracts on the market are resolved using official data from the U.S. Congress.

Odds and gambling platforms do not use methodologies used by traditional political polling, and therefore are not substitutes for political polls.

On Sunday, prediction markets gave high odds to right-wing candidate Flavio Bolsonaro to win in the Brazil presidential election before polls closed, though the incumbent left-wing president Lula da Silva was re-elected. Some markets in Democratic primaries this election cycle also drew criticism for giving sky-high odds to some candidates winning who ultimately only narrowly won or lost.

Platforms, however, say that odds traders place often end up happening that exact percentage of the time. That means, essentially, that a candidate with a 20% chance to win on an event contract exchange pulls out an "upset" win 20% of the time.

Before early September, Democrats had less than a 50% chance to sweep both chambers of Congress, though their odds have risen from around 30% earlier in the year before the start of the U.S.-Iran war and higher gas prices that have hit consumers.

On Polymarket, Democrats have a 65% chance to sweep both houses.

Polls broadly show Democrats favored in the national popular vote for the House, and competitive in a series of red-state Senate races that will decide control of the upper chamber. Though House Speaker Mike Johnson, R-La., has cast doubt on those findings.

"We've been from coast to coast," he said in an appearance on Fox News Monday, "and I'm telling you, the polling is inaccurate."

Disclosure: CNBC and Kalshi have a commercial relationship that includes customer acquisition and a minority investment.