The National Football League has entered the legal battle over prediction markets, urging the Supreme Court to let state gambling regulators treat sports event contracts the same way as traditional bets.

In a 24-page amicus brief—a type of court filing that lets outside parties with a stake in a case submit arguments—filed Thursday, the league argued that states have long regulated gambling and that sports prediction contracts constitute wagers on game outcomes rather than financial tools that help businesses hedge against losses. The filing pointed to nearly $2 billion traded on NFL games through prediction market platforms on the opening Sunday of the 2026 season—more than half of that day's total prediction market volume—and urged the justices to act "before another NFL season goes by." The Supreme Court has not agreed to hear the case, though it is widely expected to take up the matter next year.

The brief confirmed the NFL's resistance to the rapid growth of event contract platforms. The Commodity Futures Trading Commission, which regulates financial products including swaps—contracts whose payments are tied to price changes or event outcomes—has taken a largely hands-off approach to prediction markets under the current administration. That stance has allowed federally registered exchanges such as Kalshi to offer sports contracts without following traditional state gambling rules.

Citing independent research, the NFL's filing said sports contracts account for 80% of Kalshi's trading volume and that the platform recorded more than $173 billion in trades through late August 2026, compared with $25 billion across all registered exchanges in 2025.

Other major sports organizations have taken the opposite approach. Major League Baseball, Major League Soccer, and the National Hockey League have embraced prediction markets through partnerships with Kalshi and Polymarket.

"Polymarket shares the NFL's commitment to preserving the integrity of the game, which is why we … are actively collaborating with the CFTC, SEC, and other professional leagues toward a harmonized federal framework," Polymarket said in a statement.

Kalshi did not immediately respond to Fortune's request for comment.

The legal fight over prediction markets came to a head in late August, when the Ninth Circuit allowed Nevada regulators to enforce state gambling laws against Kalshi's sports contracts, creating a split among federal appeals courts. Nevada's Gaming Control Board had ordered Kalshi to stop offering the contracts without a state gaming license, while Kalshi argued that the CFTC's authority over its platform meant state gambling laws did not apply.

That ruling conflicted with the Third Circuit, which had sided with Kalshi in its dispute with New Jersey regulators in April, classifying the platform's sports prediction contracts as swaps rather than bets governed by state gambling laws.

The split widened last month: the Ninth Circuit reaffirmed its position in a separate case involving the Blue Lake Rancheria tribe, and the Sixth Circuit allowed Ohio and Tennessee to enforce their gambling laws against Kalshi's sports contracts while the lawsuits continue. In practical terms, that means whether a platform such as Kalshi faces state gambling enforcement can depend on where a dispute is filed, at least while the litigation remains unresolved.

In its filing, the NFL sided with the Sixth and Ninth Circuits, backing their position that sports prediction contracts should be treated as gambling rather than financial products. The league urged the Supreme Court to resolve the split, arguing that conflicting rulings leave oversight unclear and raise concerns about cheating, insider trading, and consumer protection. The next milestone is the justices' decision on whether to grant review; if they do, a ruling would settle the central question dividing the courts—whether sports event contracts fall under federal commodity oversight or state gambling law—in a market where sports contracts now make up the dominant share of trading.

This story was originally featured on Fortune.com.