JEFFERSON CITY — Opponents on Wednesday sued to stop a new Missouri constitutional amendment to end unlimited political giving in the state, an effort that comes as donors have continued to write six-figure checks to candidates during a monthlong window of time between the election and when the ban takes effect.

Missouri Electric Cooperatives and Legends Bank filed the federal lawsuit against the amendment, arguing it violates free speech by prohibiting contributions to political action committees from state-chartered banks, utilities and foreign corporations. The plaintiffs also had sued unsuccessfully in state court to keep the measure off the November ballot.

According to court documents, the plaintiffs claim the measure also violates their right to equal protection under the U.S. Constitution because they're banned from political giving while other companies are not.

Voters approved the measure in November, and it's set to take effect Thursday. The main goal is to limit direct political giving to candidates to $2,600 per election and giving to political parties to $25,000 per election.

Chuck Hatfield, attorney for the utilities group and bank that sued, told The Associated Press that he'll ask the court to temporarily block the amendment as the court case plays out.

The Missouri attorney general's office declined to comment.

An Associated Press analysis found at least $1.1 million in checks — each for more than $5,000 — given to state candidates during the time between the election and the amendment's enactment. The window was wealthy donors' last chance to write checks of that size directly to candidates.

Most of the money is going to successful candidates who won in November, such as Republican Gov.-elect Eric Greitens. Wealthy Joplin businessman David Humphreys on Wednesday also gave $100,000 to Republican state House Rep. Rob Vescovo.

Greitens appears to have received the bulk of donations given from the day after the Nov. 8 election through Wednesday. He received more than $460,000 in large donations in that time period, including a $20,000 check in November from the Missouri Realtors Association political action committee.

That group's lobbyist, Sam Licklider, said Greitens' campaign was not asking for money. He said the PAC gave to other candidates and also wanted to donate to Greitens, who he described as a "very nice guy."

When asked by the AP whether the donation was intentionally given before the campaign contribution limits take effect, Licklider said the PAC was "simply following the law."

"We are playing by the rules that everybody else plays by," he said.