At a campaign stop in Detroit last month, Rep. Gary Peters, the Democratic candidate for Senate in Michigan, was quick to invoke the Republican mega-donor duo Charles and David Koch, who have become the arch-villain avatars of outside money's influence in this midterm election cycle.

"As you know, we are in a very, very tough race. An awful lot of money is being spent, millions and millions of dollars have already been spent," Peters said. "... I feel like I'm not really running against [Republican] Terri Land, I feel like I'm running against the Koch brothers."

But Peters might not be complaining about the Koch brothers much longer. Their group, Freedom Partners, this week canceled its reserved television air time in Michigan for the remainder of the summer and the fall, according to one Democrat who monitors ad buys — effectively conceding the fight in a state some Republicans are cautiously optimistic could turn in their favor.

That's not to say Terri Lynn Land, the Republican candidate in Michigan, is a lost cause: Since the beginning of July, Peters has led Land in public polling by four points on average, according to Real Clear Politics poll tracking — a surmountable margin for Land. Last week, the Republican firm Harper Polling found Peters leading Land by just one point. But Land has to date posted an uneven campaign performance, while Peters has appeared to be improving.

Surveying these realities might have led Freedom Partners to decide its money would be better invested in other, more competitive states. Even without Michigan, Republicans could have multiple paths by which to net six seats, which would give the party a majority in the Senate. A spokesman for Freedom Partners did not respond to a request for comment.

But Republicans point to spending from other groups in Michigan as evidence that the party is merely reshuffling resources, not hedging its bets. As of Monday, the group Ending Spending has bought up $650,000-worth of new air time in Michigan, with more on the way, according to a Michigan Republican strategist. And the National Republican Senatorial Committee has purchased nearly $1.5 million in television advertising beginning Aug. 26, a spokesperson for the committee confirmed.

Freedom Partners is one in a tangled network of outside groups and nonprofit organizations, which includes Americans for Prosperity, steered and funded by the Koch brothers. According to a report earlier this year in Politico, Freedom Partners planned to spend $15 million boosting Republicans in this election cycle, out of $290 million by the Koch network as a whole. The ad time the group had reserved in Michigan totaled roughly $1 million.