Dark money, aka undisclosed campaign cash, played a pivotal role in Wisconsin’s recall elections of 2011 and 2012. More than half of the $90.4 million raised was undisclosed, according to a new report from the Wisconsin Democracy Campaign. And these same dark money big spenders are gearing up to splurge again in Wisconsin as the Senate race between Tammy Baldwin and Tommy Thompson heats up.

The top three issue ad groups in the recall races—Club for Growth Wisconsin, Wisconsin Manufacturers & Commerce and Americans for Prosperity—backed GOP candidates. Thanks to lax campaign finance laws, these groups do not have to reveal who funds their operations.

Club for Growth Wisconsin and Americans for Prosperity are social welfare nonprofits, classified by the IRS as 501(c)4s, that can raise unlimited amounts of cash, support and oppose candidates, and buy ads right up until Election Day.

These types of nonprofits are having a huge impact nationally, too, not just in Wisconsin. According to ProPublica, more money is being spent on TV advertising in the presidential race by social welfare nonprofits than by any other type of independent group. (Take that, super PACs.) ProPublica also reports that the majority of dark-money groups support conservative-leaning causes.

And these groups just got a lucky break this week. On Tuesday, a federal appeals court reversed a March decision that required 501(c)4s to disclose their donors if they aired specific types of ads close to Election Day. So now these nonprofits can fund whatever types of ads they want, without disclosure.

Here in Wisconsin, it’s time to brace for an onslaught of negative ads. Again. We’ll be left wondering who really paid for that ad during the Packers Monday Night Football game.

If you liked this story by Elizabeth DiNovella, the Culture Editor of The Progressive magazine, check out her story "Sherrod Brown Beats Back Big Money."

Follow Elizabeth DiNovella @lizdinovella on Twitter