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.
The lawsuit comes as donors have continued to give six-figure checks during the month-long window of time between the election and the amendment's enactment.
An Associated Press analysis found at least $1 million in checks of more than $5,000 given to state candidates during that time period