Polymarket edges Kalshi on accuracy measures for midterm chamber outcomes

According to RotoWire, Kalshi and Polymarket both correctly favored Democrats to win the House and Senate in the midterms. Polymarket had a slight advantage on the reported Brier scores for both chambers.

Prices before the results

Before polls closed, Kalshi assigned Democrats a 93.3 percent probability of winning the House, compared with 94.5 percent on Polymarket, according to RotoWire.

The Senate probabilities were lower on both platforms. Kalshi priced a Democratic win at 75.0 percent, while Polymarket put it at 75.5 percent.

By about 10 PM ET, both platforms had moved above 99 percent, RotoWire reported. The figures show the markets reaching similarly high confidence after starting with a larger gap between their House and Senate probabilities.

A narrow scoring advantage

For the House, RotoWire reported a Brier score of 0.0030 for Polymarket and 0.0045 for Kalshi. The Senate scores were 0.0600 and 0.0625, respectively, again giving Polymarket the lower figure.

The comparison therefore distinguishes between correctly favoring the eventual outcomes and the probabilities assigned to them. Both platforms got the chamber outcomes right, while Polymarket’s slightly higher pre-close probabilities corresponded with better reported scores.

What it means for prediction markets

• Outcome agreement did not produce identical scores: Both platforms favored the winning side, but Polymarket finished with lower Brier scores in both comparisons.
• The Senate carried more uncertainty: Pre-close Democratic probabilities of 75.0 percent and 75.5 percent were below the House readings of 93.3 percent and 94.5 percent.
• The evidence is limited in scope: These results compare two chamber outcomes; they do not establish broader forecasting superiority for either platform.