Jerry Lundergan's attorney told a federal jury on Tuesday that mistakes were made in reporting the contributions, but no laws were broken.
The opening arguments raised the curtain on a trial expected to last about four weeks. The charges stem from Grimes' unsuccessful 2014 campaign against Republican Sen. Mitch McConnell.
Prosecutors allege that Jerry Lundergan paid for campaign consultants and vendors through his business account without seeking reimbursement. Federal authorities say that amounted to an improper campaign contribution.