Democrats teed up a procedural vote Thursday on a bill to disclose dark-money groups' donors.
Super PACs, on the other hand, are subject to federal campaign finance disclosure laws, but their funding often comes from dark money groups.
"Unfortunately, the Republican party has become as dependent on dark money as a deep-sea diver is on his air hose," said Whitehouse.
Win McNamee/Getty ImagesA bipartisan problemSince the Citizens United ruling 12 years ago, dark money spending has exploded in elections.
Nonprofits have poured around $2 billion into elections, most of which can be linked to dark money groups, OpenSecrets found.