U.S. Sen. Angus King of Maine is calling for greater transparency around campaign financing.

King, an independent, chaired a Senate Rules Committee hearing Wednesday that examined the influence of the campaign funds known as “dark money.”

Dark money refers to campaign funding that is not disclosed to voters.

The hearing follows a recent Supreme Court ruling that removed a cap on individual contributions to federal candidates, parties and some political committees.

In his opening statement, King said undisclosed contributions threaten democracy by influencing candidates who need more and more financing.

King said a report from Wesleyan Media Project and the Center for Responsive Politics estimates $43.1 million has been spent on Senate races this election cycle.