(ONN) – The U.S. Senate race in Ohio between Democratic incumbent Sherrod Brown and Republican challenger Bernie Moreno is the most expensive race in the country and could determine the Senate balance of power.

According to Columbus based Ad Tracker media buying, Republican spending for Moreno in October has far outpaced Democratic spending for Brown’s campaign.

Finance analyst Sarah Bryner from the nonprofit Open Secrets says much of the outside money in the race is coming from dark money groups where we don’t know where the money is coming from.

“The candidates might know, the political groups involved certainly know. We don’t know then who the politician who is elected is ultimately accountable to.”