State Democrats are accusing the campaign of Rep. Rick Berg, the Republican candidate for North Dakota's U.S. Senate seat, of not properly paying for its use of a private plane Berg partially owns.

The North Dakota Democratic Party is filing a complaint with the Federal Elections Commission saying that Berg's campaign did not reimburse 714 LLP -- a limited partnership that owns the plane and consists of Berg and two partners -- for its use. Democratic Party spokeswoman Rania Batrice said Thursday afternoon that Berg's actions did not follow federal election rules requiring campaigns to reimburse owners for private plane travel, including if the candidate is a partial owner of the plane.

"It is a pretty clear cut and dry case here," Batrice said on a conference call with reporters. "If you own a private plane you have to pay fair market value."

Batrice said that the Democratic Party counted 13 trips that Berg has taken around the state on the plane during the course of the Senate race. In the complaint, which was mailed to Washington on Thursday, state Democrats detail trips Berg took out of Fargo to events in Minot, Bismarck and Manden. During these trips, Democrats say the first term congressman conducted campaign business, including attending a parade as a candidate.

Berg is challenging Democratic former state Attorney General Heidi Heitkamp in a competitive race for the seat of retiring Sen. Kent Conrad (D).

Federal election rules require a fair market rate reimbursement within a "commercially reasonable time frame" for private plane travel. Batrice said Democrats have not seen Berg report such expenditures on his federal campaign finance filing, nor had they seen a report of an in-kind contribution to the campaign from Berg and his 714 LLP partners, James Wieland and Bradley Williams.

Under questioning from the press, Batrice said the party believes that a commercially reasonable time frame would have already elapsed for the trips Berg made over the summer. Batrice said that while some of the trips occurred in July and August, the state Democratic Party only recently became aware of the extent of the trips and the lack of filings.

Berg's campaign spokesman, Chris Van Guilder, was not immediately available for comment by either phone or email.

Heitkamp's spokesman, Brandon Lorenz, told The Huffington Post that the Democrat primarily travels the state by car and chartered a plane twice during the campaign, for which he said the campaign had paid the charter company.

UPDATE: 6:45 p.m. -- Berg campaign spokesman Chris Van Guilder responded to the Democratic Party's FEC complaint by describing it as a "desperate attack" by Heitkamp and saying Berg has been compliant with federal campaign finance rules.

“This is yet another in a long line of false and increasingly desperate attacks by Heidi Heitkamp, whose campaign has not been able to explain her enthusiastic support of President Obama's big government policies, which led The Forum of Fargo-Moorhead to call her “out of step with her state” in yesterday’s endorsement of Rick Berg," Van Guilder said in an email "Our campaign regularly, and in full compliance with Federal Election Commission regulations, pays for all travel expenses, including the travel specifically noted by the North Dakota Democratic Party. We have a system in place to ensure timely reimbursement and reporting of travel expenses.”

Read the full complaint:

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