While Democratic officials and liberal advocacy groups have denounced the impact of secretive “dark money” in congressional races, new data shows that a flood of opaque left-wing campaign cash in 2018 fueled the Blue Wave that led to the Democrats’ takeover of the U.S. House.

According to a report released today by the nonpartisan reform group Issue One, the 2018 midterm elections marked the first time liberal dark money groups outspent their conservative counterparts since the U.S. Supreme Court’s upending Citizens United decision in 2010.

Overall, dark money groups, which are not required to reveal their big-bucks donors, spent approximately $150 million during the 2018 election cycle, with liberal dark money groups accounting for about 54 percent of that sum. At the same time, conservative dark money groups accounted for about 31 percent of all dark money spending, and groups classified as bipartisan or nonpartisan accounted for about 15 percent.

What’s more, one liberal dark money group, Majority Forward, accounted for about $1 of every $3 in dark money spending in 2018 congressional elections.

In Michigan, nearly $16 million in pro-Democrat funding by outside groups, including super PACs that hide their donors’ names, propelled Dems Elissa Slotkin and Haley Stevens to victory in aggressively contested House races.

The nationwide 2018 campaign numbers reflect a distinct turnabout, suggesting that Democratic and liberal groups have decided to dive in, head first, into the Wild West of campaign financing.

Issue One reports that conservative groups dominated the dark money game for several years. As recently as the 2016 election cycle, conservative dark money groups outspent liberal ones by a factor of nearly 4-to-1. Going back to 2010, the first election in the aftermath of Citizens United, conservative dark money groups outspent liberal ones by roughly 11-to-1.

Since the Citizens United court decision, which broke wide open the ability of heavily funded groups to take advantage of newly unregulated campaign cash, the total amount of dark money now approaches a cumulative $1 billion.

Based on the dollar amounts from the 2018 campaigns, the Democrats have opened themselves up to charges of hypocrisy. And Republicans are ready to double down.

“The 2018 midterms showed that Democrats are more than willing to embrace dark money,” said Rep. Zach Wamp (R-TN), co-chair of a congressional reform group.

“This should be a wake-up call to Republicans. Secret spending in elections has the potential to denigrate every candidate in every election, and candidates are losing complete control of the messages in their campaigns to these outside groups. Now is the time for Republicans and Democrats to work together to ensure that campaigns are not fought in the shadows.”

Issue One CEO Nick Penniman added this: “As we head into the 2020 presidential election, both parties must reject the opaque ways some of their wealthiest donors are influencing elections. Dark money is the most toxic force in politics. … Regardless of their party affiliation, they should have a strong personal incentive to get rid of dark money.”