WASHINGTON -- Scott Pruitt, the administrator of the Environmental Protection Agency who was key to implementing President Trump's conservative agenda but came under intense scrutiny for a series of questionable ethical decisions, resigned Thursday afternoon. Pruitt's deputy at the EPA, Andrew Wheeler, will serve as the agency's acting administrator starting Monday, President Trump said in a tweet.

"I have no doubt that Andy will continue on with our great and lasting EPA agenda," Mr. Trump tweeted. "We have made tremendous progress and the future of the EPA is very bright!"

I have accepted the resignation of Scott Pruitt as the Administrator of the Environmental Protection Agency. Within the Agency Scott has done an outstanding job, and I will always be thankful to him for this. The Senate confirmed Deputy at EPA, Andrew Wheeler, will... — Donald J. Trump (@realDonaldTrump) July 5, 2018

...on Monday assume duties as the acting Administrator of the EPA. I have no doubt that Andy will continue on with our great and lasting EPA agenda. We have made tremendous progress and the future of the EPA is very bright! — Donald J. Trump (@realDonaldTrump) July 5, 2018

Pruitt had been the subject of a seemingly endless deluge of stories about his behavior and spending practices. It began earlier this year when it was revealed that Pruitt had rented a room at a favorable rate from a well-connected energy lobbyist. Pruitt's lavish spending on his own security then came under scrutiny, as did his decision to install a $43,00 private phone booth in his office. There were also allegations that Pruitt had created an toxic professional atmosphere at the EPA that penalized his critics.

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Pruitt said his decision to leave the EPA was a hard one in his resignation letter to Mr. Trump.

"It is extremely difficult for me to cease serving you in this role first because I count it a blessing to be serving you in any capacity, but also, because of the transformative work that is occurring," Pruitt wrote. "However, the unrelenting attacks on me personally, my family, are unprecedented and have taken a sizable toll on all of us."

Mr. Trump defended Pruitt Thursday evening while speaking to reporters on Air Force One. He said there was "no final straw" and that he had not requested Pruitt's resignation.

"Scott Pruitt did an outstanding job inside of the EPA," the president said. "We've gotten rid of record breaking regulations and it's been really. You know, obviously, the controversies with Scott -- but within the agency we were extremely happy. His deputy has been with me actually a long time. He was very much an early Trump supporter. He was with us on the campaign. He is a very environmental person. He's a big believer, and he's going to do a fantastic job."

The government had launched numerous investigations and probes into Pruitt's behavior, although he continued to insist that he had done nothing wrong. At the time of Pruitt's resignation, the EPA inspector general was looking into his protective service detail, his traveling at taxpayer expense, and the allegedly excessive raises he gave to some members of his staff. Swamped by requests and stretched thin by the sheer number of audits into Pruitt, the EPA inspector general had also agreed to look into his housing arrangements and allegations that he had staff members perform his personal errands, among other issues.

The EPA inspector general said Thursday that they are now "evaluating and assessing" the status of their ongoing Pruitt audits "given the latest news," CBS News' Laura Strickler reports.

The inspector general was due to issue a report on his protective travel detail later this month. And Pruitt was also facing scrutiny from the General Accountability Office and the House Oversight Committee for various other possible infractions.

The former Oklahoma attorney general quickly became a lightning rod once he came to Washington. Early on in his tenure, he was celebrated by conservatives who saw him as a reformer unafraid to cut red tape. Liberals and progressives, however, saw him as a close ally of corporate interests who was undercutting his agency's mandate to protect the environment.

News of Pruitt's resignation was quickly welcomed by Democrats. "That this took so long shows how high the Trump administration's tolerance is for corruption and sleaze," Sen. Sheldon Whitehouse, D-Rhode Island, said in a statement. "The sad part is that it was the cascade of little sleazy acts that brought Pruitt down, not his overarching corruption by fossil fuel interests."

CREW, a progressive group, released a one word statement responding to Pruitt's resignation: "Good."

Pruitt also had Republican critics. In an interview with the Des Moines Register on Thursday before the news of Pruitt's resignation broke, Iowa Republican Sen. Joni Ernst, a frequent critic of the administrator, said Mr. Trump knew where she stood on Pruitt.

"I have obviously made my thoughts on him very clear," Ernst told the paper. "The president and again it is his decision whether to let him go or not. I would like to say his time at the EPA is short lived. But again the president knows how I feel about this."

Pruitt is the fifth member of Mr. Trump's cabinet to resign or be fired since he took office, according to CBS News' Mark Knoller. The others were former Acting Attorney General Sally Yates, former Health and Human Services secretary Tom Price, former Secretary of State Rex Tillerson, and former Veterans Affairs secretary David Shulkin.