Scott Pruitt has resigned as head of the U.S. Environmental Protection Agency amid mounting scrutiny of his spending and ethics practices.

President Trump tweeted Thursday that he has accepted Pruitt's resignation and praised the former administrator for his "outstanding" work at the agency. Deputy administrator Andrew Wheeler will become acting head of the EPA starting Monday.

Pruitt's departure follows months of negative news reports on his unusually high travel and security expenses, including a $43,000 private phone booth. Reports also show Pruitt rented a condo linked to energy lobbyists for just $50 a day and enlisted aides to do special favors, such as contacting the CEO of Chick-fil-A to help Pruitt's wife open a franchise of the restaurant.

Whistleblower Kevin Chmielewski, a longtime Trump campaign aide who served as deputy chief of staff for operations at the EPA, confirmed many of the allegations against Pruitt earlier this year. Chmielewski also leveled new claims against the administrator, including "improper retaliation against EPA staff."

There are now more than a dozen investigations into Pruitt's alleged legal and ethical violations. Earlier this week, The Washington Post reported that two of Pruitt's top aides provided fresh information to congressional investigators about some of his most controversial spending and management decisions.

According to The Post: "The Trump administration appointees described an administrator who sought a salary that topped $200,000 for his wife and accepted help from a subordinate in the job search, requested aid from senior EPA officials in a dispute with a Washington landlord and disregarded concerns about his first-class travel."

The EPA's Office of Inspector General today told CNN: “Any ongoing or pending OIG reviews related to the Administrator and/or his team will continue — regardless of the Administrator’s resignation."

Pruitt denied the allegations against him in April and has said little of the swirling scandals since then. In his resignation letter, posted by Fox News, the former Oklahoma attorney general thanked President Trump for his courage and commitment to "historical regulatory reform."

"It is extremely difficult for me to cease serving you in this role first because I count it as a blessing to be serving you in any capacity, but also because of the transformative work that is occurring," Pruitt wrote.

Pruitt, who has been widely praised by conservatives for his deregulatory agenda, attributed his departure to "unrelenting attacks" against him and his family. The EPA administrator's resignation comes days after a teacher confronted him at a Washington, D.C. restaurant to express her disapproval of his policies and urged him to resign.

During his tenure, Pruitt oversaw the largest regulatory rollback in EPA history. The New York Times reported that he delayed or blocked more than 30 environmental rules within his first four months of taking office, including the Obama administration's Clean Power Plan and an Obama-era rule curbing pollution in waterways. As of last October, 67 environmental rules appeared to be under threat.

Pruitt's EPA has also put restrictions on environmental science and is in the process of weakening federal fuel economy standards.

Under Andrew Wheeler, the EPA is unlikely to change course. Wheeler is a former aide to Republican Senator James Inhofe, an outspoken climate change skeptic, and a former coal industry lobbyist. He also urged the Department of Interior to reduce the size of Bears Ears National Monument to make more land available for uranium mining.

Fred Krupp, president of the Environmental Defense Fund, said the EPA will continue to put the health of American families in jeopardy under Wheeler's leadership.

"We now face the stark reality of a coal lobbyist, Andrew Wheeler, running the agency that is supposed to protect our air, water and climate," he said, in a statement. "We can't afford more pollution and asthma attacks — our children's health and their future depends on it."

As a Washington insider, Wheeler is well positioned to enact regulatory rollbacks, which is a priority for this administration. President Trump tweeted Thursday, "I have no doubt that Andy will continue on with our great and lasting EPA agenda."

Brandon Hurlbut, former chief of staff at the Department of Energy, said the ethics complaints are one thing that is likely to change with Wheeler. Hurlbut, who has first-hand knowledge of Wheeler's work, said he disagrees with the acting EPA chair on policy matters, but acknowledged that Wheeler is generally well respected.

"Pruitt abused power in a disgraceful way by using the job for perks and trying to enrich himself and his family," Hurlbut told GTM. "While I disagree with Andrew Wheeler on many environmental policies, particularly coal, I know him personally as someone with high ethical standards and don't expect him to engage in pervasive corruption like Pruitt."

Wheeler has largely stayed out of the limelight and previously told the Washington Examiner he has no interest in taking the EPA administrator role. Other names have been floated as possible Pruitt successors, including Donald van der Vaart, a former environmental official in North Carolina.

For additional commentary on Pruitt and Wheeler, listen to this recent episode of GTM's newest podcast, Political Climate.