As dark money spending breaks all records this election season, corporate-backed groups are trying to strike down one of the strongest state-level dark money transparency laws in the country in a key swing state.

Last year, voters in Arizona overwhelmingly passed a ballot measure — Prop 211 — that requires dark money groups funding political advertising in the state to reveal the original source of their money. The unprecedented law is now one of the most far-reaching dark money transparency laws in the country.

It has also become a target for corporate interests trying to limit campaign finance laws. Last year, Americans for Prosperity, a nonprofit backed by billionaire Charles Koch and his shadowy dark money network, sued in federal court to overturn Prop 211. Now, the group has appealed the case to the US Court of Appeals for the Ninth Circuit, a move likely designed to get the matter before the Supreme Court.