The Benjamin Pollard for Senate committee should have filed its report no later than May 31 for financial activity from April 1 through May 12, the FEC said Friday in a release.
Maine's party primaries are Tuesday. The state's party caucuses were in February.
Pollard is one of several Democrats seeking to be the party's nominee in the race to succeed retiring U.S. Sen. Olympia Snowe, R-Maine.
Some individuals and their committees aren't obligated to file reports under federal campaign finance law, even though their names may appear on state ballots, the watchdog agency said. If an individual raises or spends $5,000 or less, the person isn't considered a "candidate" subject to reporting under the Federal Election Campaign Act of 1971.