North Carolina’s Democratic Sen. Kay Hagan and her Republican challenger, state House Speaker Thom Tillis, are preparing for a Tuesday night debate in a contest that’s shaping up as one of the most red hot in the nation: An analysis of Federal Elections Commission data by Sunlight’s Real-Time Federal Campaign Finance tracker shows that the Tar Heel state contest is attracting more outside cash than any other Senate race.

Of the $37.5 million that outside groups have dumped into North Carolina so far, $25.4 million has gone for negative advertising. More than $6.5 million has gone for ads opposing Hagan and a jaw-dropping $18.8 million for ads opposing Tillis.

Among the 35 outside organizations that Sunlight has identified as active in the North Carolina race, the biggest spender by far is the Senate Majority PAC, a super PAC directed by former staffers for Senate Majority Leader Harry Reid, D-Nev. It has so far pumped more than which so far has put more than $10 million dollars in the race, more than twice as much as the official Democratic Senatorial Campaign Committee.

Outside spending in North Carolina’s Senate race

One of the Crossroads groups, Crossroads GPS, is a dark money group, a term Sunlight has coined to identify nonprofit organizations that don’t register with the Federal Election Commission and don’t disclose donors but raise and spend money to try to influence elections.

The existence of such organizations means that the $37.5 million we have identified in North Carolina represents only a part of the outside spending there.

The campaign-style activities undertaken by dark money groups never have to be reported to the Federal Election Commission as long as they take place more than 60 days before the general election and don’t directly ask for a vote for or against a candidate. There is a paper trail, however, for broadcast television ads. Using Sunlight’s tools for tracking political ad buys, Political Ad Sleuth, we have identified a number of buys during the spring and summer by outside groups that directly targeted one or the other of the candidates. Most of the ads that we have found running outside the reporting window from dark money groups appear to have targeted Hagan.

For instance, the 60 Plus Association purchased airtime in several North Carolina markets to air an anti-Hagan ad. Note that it does not recommend a vote for or against Hagan — words that would have required the expenditure to be reported to the FEC.

One of the most active outside spending groups in North Carolina during the spring and summer: Americans for Prosperity, a group funded by the conservative brothers Charles and David Koch, which appears to have stopped spending as soon as the 60 day pre-election reporting window opened. Two months before a general election, any ad that mentions or depicts a candidate for office must be reported to the FEC. Here’s a sample of the type of anti-Hagan ads AFP was airing during the summer:

One of the few pro-Hagan ads aired by a nonprofit, the Southern Alliance for Clean Energy buy, directly took on the Koch brothers for attacking her:

Totaling how much money these organizations spent during the summer is difficult because it would require hand-entry of hundreds of filings at the Federal Communications Commission. And even that would not necessarily completely capture dark money spending that went for activities other than TV ads.

One thing is clear, however: A lot of outside interest groups have a lot invested in who is going to win the Senate race in North Carolina.