Former Inhofe aide Wheeler confirmed as EPA's No. 2

If Pruitt left, it would fall to Wheeler to run the agency until a new administrator is confirmed.

His firm's clients include Murray Energy, which bills itself as "the largest coal mining company in America."

Senate Minority Leader Chuck Schumer, D-New York, previously blasted Wheeler as "a former industry lobbyist who has worked on behalf of big polluters and climate change deniers. He has spent years working to undermine or lobby against the environmental protections he may soon oversee."

Prior to his lobbying work, Wheeler served on Capitol Hill as a Republican staff member for the Senate Environment and Public Works Committee and as a top aide to Sen. Jim Inhofe, an Oklahoma Republican and an outspoken climate change skeptic who told CNN last month the EPA is "brainwashing our kids."

New Mexico Democratic Sen. Tom Udall said Wednesday that Wheeler should be carefully vetted, as if he were taking over for Pruitt now. Udall told reporters that he believes Wheeler wouldn't be a much better option to run the agency.

"The problem with the Wheeler nomination is if Trump (fires Pruitt) tomorrow, Wheeler is in fact the administrator, and that is a very, very serious problem," Udall said. "I know that there are many Republicans who haven't spoken out yet, but privately they are very disturbed by what Scott Pruitt is doing at the EPA."

Wheeler's nomination comes amid a steady stream of negative headlines involving Pruitt in recent weeks and months that has official Washington wondering whether the embattled agency chief can hold onto his job.

Most recently, Pruitt has been fighting stories revealing he paid about $6,100 over the course of a six-month lease last year to rent a room in a condo owned by Vicki Hart, a health care lobbyist whose husband, Steven Hart, has lobbied the EPA. His daughter also reportedly lived there while she was interning in Washington.

He paid $50 per night, according to the reports, and paid only for the nights he used the condo.

The federal government's top voice on ethics David Apol sent a letter to the agency outlining areas of concern regarding Pruitt. Apol, the acting director and general counsel of the Office of Government Ethics, summarized reports of Pruitt's conduct, including the rental agreement, as well as EPA spending on Pruitt's travel and security.

The letter also expresses concern with reports of Pruitt bypassing the White House to give raises to favored aides and other employees who faced job changes after raising concerns over his conduct.