Correction: An earlier version of this story misidentified the office held by Ohio State Treasurer Josh Mandel. The story has been corrected.

Sen. Sherrod Brown speaks in Columbus, Ohio, on May 12. (Mark Duncan/AP)

In the battle for control of the Senate, no race has received as much financial attention as the reelection bid of Democratic Sen. Sherrod Brown, with outside conservative groups working together to pour tens of millions of dollars into Ohio’s airwaves to try to unseat him.

In early July, the U.S. Chamber of Commerce launched ads criticizing the liberal senator for alleged anti-business votes. When that run ended, a nonprofit group affiliated with GOP operative Karl Rove hit Brown with a new slate of ads. Those were immediately followed by another set of chamber ads denouncing Brown as a “career politician.”

The conservative groups, led by the business-friendly chamber, have spent more than $11.5 million in a relentless anti-Brown campaign — that’s more than twice as much money as Brown’s actual opponent, Ohio State Treasurer Josh Mandel, has spent on the race. The chamber has vowed to spend much more, and Crossroads GPS, the Rove-affiliated group, already has reserved an additional $6.7 million in advertising for the final five weeks of the contest.

The Brown-Mandel campaign is a case study of the aggressive fundraising and spending this election season by interest groups outside the candidates’ campaign operations. And because many of the groups behind the spending are not required to disclose their donors, the effort has created a virtual shadow campaign that will probably far exceed what Mandel spends on his campaign

Liberal in a swing state

Public polling has given Brown, 59, who has been in Congress for two decades, a modest lead over Mandel, 34, despite the deluge of outside spending. But the race has shown signs of tightening even as his less experienced rival has suffered from missteps on the campaign trail. To the incumbent, there’s a simple reason this race is expected to go down to the wire.

“This would not be a race if he didn’t have the 111 / -and-a-half million in outside money spent. It wouldn’t even be a race. That’s what outside money does,” Brown said in an interview outside the Capitol.

Brown said he expects to face a total of nearly $30 million in outside spending, an amount that will most likely dwarf Mandel’s entire campaign war chest.

Why Brown? He is the most unabashedly liberal senator who hails from a swing state. He recently introduced legislation to limit the size of mega-banks. He introduced legislation to end subsidies to the oil and gas industry. In 2010, he earned a 9 percent voting record as compiled by the chamber.

His is just one of up to a dozen Senate races where the groups, some taking advantage of the 2010 Supreme Court ruling that loosened campaign finance law, figure to play key roles. The chamber, which does not engage in presidential politics, does not reveal its political budget, but Democratic and Republican officials who have monitored the group expect this year’s spending on congressional races to about triple its 2008 budget of more than $30 million.

“The chamber is launching its largest and most aggressive voter education campaign in our 100-year history to draw a clear choice between candidates who support the free enterprise system and those who consistently support more government,” said Scott Reed, the top political strategist for the organization.

Early last year, conservative leaders began a concerted effort to flip the Senate into Republican hands. Holding a 53 to 47 edge, Democrats headed into the 2012 campaign knowing they had to defend 23 seats to just 10 seat for Republicans in November. The outside conservative network — whose main players are the chamber, Crossroads and a collection of groups connected to the billionaire industrialist Koch brothers, David and Charles — decided it needed to expand the map to create as many possible routes to the net gain of four seats for the Senate GOP, according to participants.

This has meant that, beginning last November, Brown has been under nearly nonstop fire. Since then, viewers in the critical swing media market of Columbus have seen more than 30 weeks of TV ads critical of the incumbent, according to Democrats monitoring Ohio’s media markets. In May, when Sen. Bill Nelson (D-Fla.) seemed to have a comfortable lead, the chamber dropped an estimated $2 million into ads that accused him of casting the “deciding vote” for President Obama’s health-care legislation. After Nelson’s race grew closer following the chamber’s advertising, other groups jumped into the Sunshine State this month. Crossroads has reserved more than $6 million worth of ads for the fall race.

More so than super PACs — whose unlimited donations must be reported and which dominated the Republican presidential primary — nonprofit groups that do not reveal their donors are holding great sway over congressional campaigns. They are not allowed to coordinate their activities with the candidates or the official party campaign committees, but they do coordinate among themselves. Crossroads, which is run by the former chief of staff to Senate Minority Leader Mitch McConnell (R-Ky.), hosts a monthly meeting of groups working on the fall campaign.

According to participants, however, there are biweekly meetings of a smaller group of the most influential players, at which truly big decisions are made. The chamber and Crossroads do most of the big advertising, with the potential for spending $200 million combined in congressional campaigns this year. Americans for Prosperity, whose finances flow from the Koch brothers, does much of the polling, hosts focus groups and runs grass-roots work in the states, sometimes sharing its findings with the other two groups.

A busy U.S. Chamber

Senate Democrats have a super PAC, called Majority PAC, staffed with former aides to Senate Majority Leader Harry M. Reid (Nev.) and Charles E. Schumer (N.Y.), but it has not been able to keep pace with its conservative counterpart. Majority PAC has spent $1.6 million in Ohio, mostly in attack ads against Mandel, but that’s a fraction of what Brown has faced from conservative groups.

The conservatives also have excess dollars because of a pact between Sen. Scott Brown (R-Mass.) and Harvard professor Elizabeth Warren (D) to keep interest group money out of their race. Cash that might have been spent against Warren has now been freed up for other races on the margin.

Late last month, the chamber jumped into Maine with a 10-day purchase of ads criticizing Angus King, the popular former governor who is running as an independent. It has targeted other inexpensive media markets, such as New Mexico and Hawaii, places where the Democrat is favored but Republicans have good nominees.

Republicans and Democrats dispute the legitimacy of the outside money. Republicans said the conservative cash is leveling the playing field against what had been millions of dollars of undisclosed spending by labor unions.

“We have been operating, essentially, at a huge disadvantage,” said Sen. John Cornyn (Tex.), chairman of the National Republican Senatorial Committee, the chief strategist for GOP Senate races. “I’m one who happens to believe more speech and more information about the candidates and the issues is a good thing, not a bad thing.”

An arm of the GOP?

Democrats are particularly dismayed by the chamber siding so heavily with Republicans, considering the organization’s brand as representing Main Street. The chamber has not yet endorsed a single Democrat in a remotely competitive Senate race.

“They basically have become an arm of the Republican Party, and that’s not good for the businesses they represent, but that’s what is happening. The local chambers don’t back that up,” said Schumer, the No. 3 Democrat in the Senate. “Instead of sharp shooting, they’re firing every howitzer around.”

No candidate has faced the barrage quite like Sherrod Brown. In 2008, the chamber’s most expensive race was the reported $4.3 million spent in New Hampshire. The group topped that figure last month when its most recent ad campaign against Brown brought the total tally to more than $4.7 million.

Brown is frustrated because the money is coming from undisclosed sources, but he said that he has a pretty good idea who is funding the effort. “It could be a few billionaires, it could be Exxon. I assume, because of my record, it’s Wall Street banks. I assume it’s Exxon and other oil interests,” he said.

“It makes some people think, ‘If I vote on this amendment, man, they’re coming after me,’ ” Brown said of the heavy spending. “But they’re coming after everybody who has a D after their name.”