ST. LOUIS (AP) – Democratic U.S. Senate candidate Jason Kander, who is involved in a tight race with incumbent Republican Roy Blunt in Missouri, has written a $25,000 check to the U.S. Treasury as a way of turning back questionable campaign donations.

A report in the Boston Globe found that the Thornton Law Firm in Massachusetts allegedly reimbursed partners for political donations, a practice that is illegal under federal election law. Multiple donations from the firm amounted to $25,000 to Kander’s campaign, all on May 14, 2015.

Kander spokesman Chris Hayden says the campaign became aware of the situation Sunday and immediately gave the money to the Treasury.

Recent polls show Senate race in Missouri is a virtual tossup.