Secret money from outside groups is flooding election campaign coffers at the highest level in 16 years to nearly a third of all political advertising, a study found.

Super PACs make up the bulk of the ad buys in the 2016 election cycle, accounting for nearly 73 percent of all outside group spending, according to the study by the Wesleyan Media Project and the Center for Responsive Politics.

[Super PAC Announces $10 Million to Support House GOP Candidates]

The study, which tracked more than 700 organizations behind campaign ads between 2000 and 2016, found that while candidates are responsible for the majority of ads in a particular election cycle, a few competitive House and Senate races are seeing more spending by outside groups than by those running for office.

[Top GOP Super PAC Ups Spending in Senate Races]

Supreme Court decisions that have loosened campaign finance laws have allowed politically active nonprofits to drive the growth of “dark money” largely linked to wealthy donors or political operatives, the study showed.

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