Opponents are suing to stop a new Missouri constitutional amendment to limit political giving.
Missouri Electric Cooperatives and Legends Bank on Wednesday filed a federal lawsuit against the amendment, which takes effect Thursday.
Voters in November approved capping direct contributions to candidates at $2,600 per election. Donations are limited to $25,000 for political parties.
Attorney Chuck Hatfield said he plans to ask the court to temporarily block the amendment as the court case plays out.
The attorney general's office did not immediately comment.