The secretive political network of conservative billionaires Charles and David Koch has aired more than 43,900 television ads this election cycle in an attempt to help Republicans take control of the Senate in the upcoming November election. That amounts to nearly 1 out of every 10 TV ads in the 2014 battle for the Senate, according to a new Center for Public Integrity analysis of data provided by Kantar Media/CMAG, an advertising tracking service, covering spending from Jan. 1, 2013, through Sunday.

The total includes the six most active nonprofit groups in the Koch brothers’ coalition: Americans for Prosperity, the American Energy Alliance, Concerned Veterans for America, the Freedom Partners Chamber of Commerce, Generation Opportunity, and the 60 Plus Association.

Their prominence has led to denunciations by Democrats, and praise from Republicans, as they’ve bombarded incumbent lawmakers with negative ads and exulted conservative challengers. No other right-leaning coalition has been as active. Even the two main big-money committees co-founded by GOP strategist Karl Rove—American Crossroads and Crossroads GPS—have only aired about half as many ads to date as these six Koch-connected groups. In all, Americans for Prosperity, the Koch brothers’ flagship political operation, alone has aired more than 27,000 ads in a combined nine battleground states, according to Kantar Media/CMAG.

Koch-connected groups reportedly intend to spend $290 million to help Republicans make gains in Congress this November. Thus far, Kantar Media/CMAG spending estimates indicate the groups have invested at least $14.5 million. That amount is undeniably a conservative estimate, as it includes only TV ad buys—not production costs or expenditures related to radio ads, online ads, direct mail, canvassers, or other activities. These so-called “dark money” nonprofit groups are not required to disclose their funders to federal election regulators, unlike candidates, parties, political action committees, and super PACs. And although election-related advocacy can’t be the “primary purpose” of these groups, they’ve nonetheless established themselves among the nation’s most powerful political forces.

In Alaska, the Koch-backed Freedom Partners Chamber of Commerce has accused Democratic Sen. Mark Begich of “standing with insurance companies” by supporting President Barack Obama’s health care reform law. Meanwhile, Generation Opportunity, a Koch group focused on millennials, has argued that Democratic Sens. Mark Udall of Colorado and Kay Hagan of North Carolina are “forcing” young adults to “buy health insurance.” And Democrat Gary Peters has been slammed by Americans for Prosperity for supporting Obamacare, which the group contended was “making things worse” for Michigan families.

The top six Koch-connected nonprofits have hammered Democratic Senate candidates on an ever-expanding geographic field. Through the end of August, this spending spree has included about 8,600 ads in North Carolina, 6,900 ads in Louisiana, 5,800 ads in Iowa, 4,900 ads in Michigan, 4,700 ads in Arkansas, 4,600 ads in Colorado, 3,600 ads in Alaska, and 2,400 ads in Oregon, according to a Center for Public Integrity review of Kantar Media/CMAG data.

Democrats have responded in kind, using secretive nonprofit vehicles of their own. They’re led by big-spending Patriot Majority USA, a “social welfare” nonprofit run by a close ally of Senate Majority Leader Harry Reid—a pre-eminent Koch brothers naysayer who even maintains an anti-Koch page on his official Senate website.

Bill Allison, editorial director of the Sunlight Foundation, which advocates for transparency in government and tracks political advertising, said the Koch network is trying to make the Democratic candidates “unacceptable to voters before Labor Day.” Allison continued: “You only get one chance to make a first impression.”

When TV viewers see these ads, said Louisiana State University political science professor Johanna Dunaway, they may not yet be “primed enough to recognize” the names or agendas of these conservative groups. Her own research has found that “ads sponsored by unknown interest groups are more persuasive than those sponsored by candidates or known interest groups.” These groups’ names sound “benign and credible,” said Dunaway.

“There’s a real advantage to defining your candidate positively and your opponent negatively as early as possible,” added Chris Mann, a political science professor at Louisiana State University. Ads from noncandidate groups, Mann continued, can help “offset the incumbent advantage.”

In more than a dozen states, the Democratic Party finds itself on the defensive, as Republicans seek to wrest control of the U.S. Senate away from them. To do so, the GOP must pick up six seats from Democrats, whose candidates, generally, have amassed a financial advantage and whose allies have fought back with TV advertisements of their own. As of midsummer, the five incumbent Democratic senators in the most contested re-election races had collectively raised more than $60 million, according to federal campaign finance filings. Their GOP opponents had collectively raised about $30 million.

The most prolific of these fundraisers was Hagan of North Carolina, who, as of the end of June, had raised nearly $17 million—about 3½ times the amount raised by her GOP opponent, Thom Tillis. Meanwhile, both Udall of Colorado and Alaska’s Begich had each raised between double and triple the amounts of their Republican rivals. As of June 30, Udall had raised nearly $14 million, compared with about $5 million raised by Republican Cory Gardner. And Begich had collected roughly $8 million as of July 30, compared with about $4 million by Republican Dan Sullivan, who won a contested GOP Senate primary in late August.

Koch-linked outside groups have helped Republicans cultivate support—in a profound manner. In an interview with C-SPAN last month, Tim Phillips, the president of Americans for Prosperity, acknowledged that “it’s very difficult to beat a sitting United States senator.”

“The incumbent Democrat senators who are up, their only chance is to make this about personalities or politics,” he continued. “If it's on the issues or their performance, they're probably goners.”

In these battleground states, many Republicans are grateful for the outside assistance. Kermit Parks, chairman of Union County Republican Party in Arkansas, called the anti-Obamacare ads from Americans for Prosperity that criticize incumbent Democratic Sen. Mark Pryor “money well spent.” Steve Lux, chairman of the Saline County Republican Party in Arkansas, agrees. “Obviously everybody says they hate negative ads, but they do have an effect,” Lux said.

Democrats, though, remain defiant. “Sen. Pryor has withstood this unprecedented onslaught and remains in a good position to win this race,” campaign spokesman Erik Dorey said. These big-money conservative groups, Dorey added, “don’t have Arkansans’ best interests at heart.”

This story was published by the Center for Public Integrity, a nonprofit, nonpartisan investigative news organization in Washington, D.C.