Outside groups spent nearly $25 million to reelect Ohio GOP Sen. Rob Portman.. (AP Photo/J. Scott Applewhite)

Outside groups have poured roughly $95.5 million through Aug. 30 into five Senate races aimed at the general election, proving that donors are deeply invested in the fight over which party controls Congress in 2017.

The groups are spending big in Pennsylvania, Ohio, Nevada, New Hampshire, and Wisconsin. All of the races except Nevada are places where where Republican incumbents are battling well-known, popular Democrats in states that have historically tipped blue in presidential elections. Republicans hold a slim 54-48 seat majority in the Senate and the races represent some of the toughest battlegrounds in this election.

The nearly $100 million price tag is according to groups who report any ad buys or other activities such as direct mail explicitly supporting or opposing the election of a candidate. Non-profits like the ones associated with the Koch network which run issue based ads are not required to disclose with the FEC. Such non-profits do not need to disclose their donors and are rising in influence in the 2016 elections.

A new analysis released Wednesday by the Brennan Center estimated that “shadow party” groups with close ties to Senate leaders have spent $47 million so far this cycle on Senate elections. These groups also are not required to reveal their donors.

The center said that three major coalitions — Democratic-allied interests; the U.S. Chamber of Commerce and the Koch network — were responsible for 58 percent of spending in the eight most competitive Senate races.

The two races with the most concentrated amount of outside money being spent on them are Ohio, where Republican Sen. Rob Portman is facing off against former Ohio Gov. Ted Strickland (D); and Pennsylvania, where Republican Sen. Pat Toomey faces Democrat Katie McGinty. Outside groups have spent more than $30 million in each of those states.

In Ohio, outside groups have spent $39.8 million with more than $24.4 million invested in backing Portman’s reelection bid.

A recent poll from Emerson College shows Portman leading by 15 points, his most generous lead in months. He has benefited from significant ad support from groups like Freedom Partners Action Fund, Fighting for Ohio Fund and the U.S. Chamber of Commerce.Groups backing Strickland including labor groups like AFSCME and SEIU have spent $7.5 million.

U.S. Chamber of Commerce senior vice president and national political director Rob Engstrom said Wednesday that the group has been dedicated to defending Portman since last July and the latest number represent that commitment.

“The clock is now ticking in the 4th quarter,” Engstrom said. “Candidates who support defending American free enterprise will find no greater friend. Now is the time to close, ground game matters.”

Outside spending in neighboring Pennsylvania — where Republican Toomey is trailing McGinty — has reached a total of more than $30 million so far.

Toomey, who co-authored a failed 2013 gun control measure, has benefited from $3.1 million in spending from Independence USA, the super PAC mainly funded by former New York City Mayor Michael Bloomberg. The Koch brothers-backed group Freedom Partners has also shelled out $5.6 million in ads attacking McGinty.

McGinty, however, has gotten a major assist from labor unions, Planned Parenthood and Senate Majority PAC, which is backed by Senate Minority Leader Harry Reid (D-Nev.).

Republican strategists credit some of the eye-popping spending to the relatively high cost of buying ads in Pennsylvania and Ohio. Media markets around cities like Philadelphia, Pittsburgh, Cleveland and Columbus drive up the price of ads far higher than in states with more rural populations.

The only race in the top five where a Republican isn’t on the defense is the open seat of the retiring Reid in Nevada. Democrat Catherine Cortez Masto and Republican Joe Heck have seen a total of $14.6 million in outside spending.

Over a third of the total money, $4.5 million, was spent by Koch-backed Freedom Partners on ad buys against Masto. Another $2.1 million came from the National Republican Senatorial Committee.The outside group spending drops off steeply for the remaining two races.

Groups have spent approximately $6.8 million in New Hampshire where Republican Sen. Kelly Ayotte has been fighting neck-and-neck with Democratic Gov. Maggie Hassan. Spending in Wisconsin — where Republican Sen. Ron Johnson trails former Sen. Russ Feingold by 11 points — comes in at $4.3 million.