MADISON (WKOW) -- Ads bought and paid for by third-party political groups trying to influence voters in the race between Senator Ron Johnson (R-Wisconsin) and challenger Russ Feingold (D) are already clogging up radio and TV commercial slots across the state.
"That's the sorry state of politics in America today. We're drowning in a sea of dark money," Wisconsin Democracy Campaign Executive Director Matt Rothschild told 27 News.
Rothschild's group exists to shine a light on dark money in politics, but even they struggle to find it, as do others.
The non-profit investigative journalism group ProPublica is tracking third-party spending in every key U.S. Senate race this year.
Their current numbers show a huge advantage in ads being run on behalf of Sen. Johnson or against Feingold.
"I think you've seen millions of dollars coming in from out of state, from a couple of oil billionaires from Kansas on behalf of Senator Johnson and I hope we don't see more of that, but I'm expecting we probably will," said Tom Russell, Feingold's campaign director.
But that's only half of the story, because ProPublica and other groups can't actually track all of the third-party money being spent in the race.
"The simple explanation is - they don't have to disclose it," said Rothschild.
Many of the groups supporting Sen. Johnson are structured in a way where they don't have to disclose their donors, but do have to disclose their spending.
But other groups, like the League of Conservation Voters (LCV) - which is heavily backing Feingold - have arms specifically structured to avoid even disclosing their spending.
"They don't have to file - these issue advocacy groups - with anyone except the IRS," explained Rothschild. "They have to file a so-called 990 form, but they don't have to file it for more than a year after the election."
In other words, there is no way to know now for sure just how much is being spent unless the groups themselves choose to disclose it.
Making his name as a champion of campaign finance reform during his 18-year career in the Senate, Feingold has taken heat from the Johnson campaign for not distancing himself enough from groups like the LCV, who he's actually appeared with at events as recently as May.
The Johnson Campaign would not give 27 News an interview for this story, but a Republican Party of Wisconsin spokesperson calls Feingold a hypocrite.
"He's railed against outside spending and dark money for years, when he's actually directly benefiting from that type of campaign spending," said Katie McCallum, fourth vice-chair at the RPW.
"Our response is a) the Badger Pledge and that we've actually made an effort, a real effort to try and keep these groups out of the race in a bipartisan way," said Russell.
In early 2015, Feingold asked Johnson to sign the so-called Badger Pledge - an agreement to prohibit any third-party money from entering into the race,
"And the way to do it is the way they did it in Massachusetts with Senator Warren and Senator Brown," said Feingold, referring to the 2012 election there. "They said look - if one of these groups comes in and spends on your behalf - the deal is that you have to pay half of the value of those ads to your opponent's favorite charity. It worked there, it would work here. Senator Johnson didn't respond because he believes in this system."
"You know, he's not kept his word on this issue and how do you enter into an agreement with someone like that," responded McCallum. "It's a non-starter."
The Johnson campaign sent 27 News a statement in response to Feingold's comments on the Badger Pledge.
"His Badger Pledge was a hypocritical effort to deflect attention away from all the money Progressives United raised to fund his personal slush fund and build his current fundraising apparatus. Until he explains himself to Wisconsinites, there's nothing left to talk about," wrote Johnson Spokesperson Brian Reisinger, referring to Feingold's former political action committee started in 2011 and later shut down in 2015 .
There was close to $50 million in outside money spent in the 2012 Senate race between Democrat Tammy Baldwin and Republican Tommy Thompson.
The final figures in this race are expected to be similar.