Environmental Protection Agency Administrator Scott Pruitt said Thursday that he won't be headlining a Republican fundraiser after a top Democratic critic of his in the Senate leveled an ethics complaint.

The EPA ethics office "actually approved my attendance," Pruitt explained on Fox News Radio's Kilmeade and Friends. But the way the invitation was sent out "didn't comply with federal law and federal ethics law, so we're not going to be able to attend because of the invitation," he explained.

Pruitt was invited to attend an Oklahoma Republican Party fundraiser next week, where he was supposed to headline the event and give the keynote address.

But Sen. Sheldon Whitehouse, D-R.I., found out about the invitation and alerted the U.S. General Counsel's office, lodging a formal complaint that Pruitt may have violated the Hatch Act, which prohibits senior Cabinet officials from participating in certain political activities. Whitehouse's complaint said the invitation made it appear as it the money donated to the GOP was contingent on Pruitt's appearance.

Whitehouse said Thursday that Pruitt's decision to skip the event is "the least he can do." The senator, who is a top opponent of Pruitt's appointment by President Trump, said the counsel's office still must conduct a "thorough investigation."

Whitehouse said in a statement that Pruitt "has a long record of dark money fundraising and cozy relationships with big, fossil-fuel political donors.

"The American people need to know whether he is using his position at EPA to promote the political actors who support him," he said.

Whitehouse, and a number of other Democrats on the environment committee, which oversees the agency, attempted to hold up the Senate's vote to approve Pruitt earlier this year. They call Pruitt a supporter of polluters and the fossil fuel industry.

They cited the need to review all communications between Pruitt and energy firms while he was attorney general of Oklahoma. The Democrats continue to fight Pruitt's implementation of the president's executive orders to roll back a number of regulations, including former President Barack Obama's climate change rules.