ST. PAUL — A group challenging a Minnesota campaign finance law asked a federal judge Monday to suspend a cap on some campaign contributions.

The Minnesota branch of the Institute for Justice contends that “special source” limits on how much a candidate can take from lobbyists, wealthy donors and political funds have a chilling effect on free speech.

Currently, state law limits the total amount Minnesota candidates can take from certain types of contributors. The two donors and two candidates who filed the challenge contend the law sets up a first-come, first-serve system and infringes on the right of donors to give more. The law was designed to prevent candidates from focusing too much on big-dollar donors.

For example, Minnesota House candidates can take up to $1,000 from contributors. But once they hit $12,500 in donations from those “special sources” who give between $500 and $1,000, everyone who contributes to the campaign after that can only give $500.

“The government can’t say that only the first 12 people in line get to contribute $1,000 to a candidate, while everyone else only gets to contribute half that,” Institute for Justice attorney Anthony Sanders said.

But Minnesota Solicitor General Alan Gilbert said the plaintiffs haven’t proved the rules prevent free speech.

“(The donors) can contribute today,” said Gilbert at the hearing. “They can contribute $1,000.”

Gilbert added that Republican state Rep. Linda Runbeck of Circle Pines, who is one of those challenging the cap, has chosen to ask donors to give $500 or less even though she can accept $1,000.

The Institute for Justice filed its lawsuit shortly after the U.S. Supreme Court struck down the total cap on how much individuals can give to candidates, parties and political action committees.

In Minnesota, donors can give the maximum amount to as many candidates as they choose.

U.S. District Judge Donovan Frank said he’ll decide soon on the case.