Combined trading volume across Kalshi, Polymarket and Polymarket US fell 14.5% in August to $45.33 billion, the first month-over-month decline since August 2025, after a summer peak driven by the FIFA World Cup from June 11 to July 19. Kalshi recorded $37.17 billion, down 7.3% from $40.1 billion in July, while Polymarket and its U.S. platform posted $8.16 billion, a 36.7% drop from $12.89 billion; August still exceeded May's $25.66 billion. Both platforms face mounting legal pressure, with more than a dozen states taking enforcement action or filing lawsuits largely over sports-related contracts, including Connecticut's Aug. 27 suit seeking to block Kalshi from offering sports contracts. Kalshi still expanded its sports presence as the exclusive prediction market partner of the US Open under a U.S. Tennis Association deal as the main draw began Aug. 30, and on Aug. 31 issued its first permanent ban, removing former U.S. Representative George Santos for a prohibited State of the Union attendance bet and ordering a fine exceeding $71,000. Volumes remain well above pre-tournament levels even as the post-World Cup lull and regulatory fights frame the near-term outlook.