ST. PAUL, Minn. (AP) — Minnesota campaign finance law that restricts how much candidates can take from certain donors is being challenged in federal court.

The case filed Wednesday by the Institute for Justice on behalf of two donors and two candidates aims to scrap the "special source" limitation. That section of campaign law reduces the maximum checks candidates can accept once they collect predetermined amounts from big donors, political funds and lobbyists.

The plaintiffs say it sets up a first-come, first-serve system and infringes on rights of donors to give more. The law was designed to prevent candidates from focusing too much on big-dollar donors.

Plaintiffs want an injunction barring enforcement of the law this election year.